Accuracy of portable ultrasound for obstetric biometry.

OBJECTIVE: We assessed the accuracy of two portable ultrasound machines (PUM) in obtaining fetal biometry and estimating gestational age. METHODS: We analyzed data from the Fetal Age Machine Learning Initiative, an observational study of pregnant women in the United States and Zambia. Each participant underwent assessment by an experienced sonographer using both a high-specification ultrasound machine (HSUM) and a PUM (either Butterfly iQ or Clarius C3) to measure fetal biometry and calculate estimated gestational age (EGA) at each visit. Through comparison of paired PUM-HSUM scans, we estimated agreement between individual biometry measurements and aggregate gestational age estimates by reporting mean difference, along with intraclass correlation coefficient (ICC) and Bland-Altman plots, adjusting for trend. RESULTS: 881 participants contributed 1386 paired PUM-HSUM ultrasound studies between April and December 2021. PUM studies included 991 Butterfly and 395 Clarius. Gestational age at scan ranged from 7 to 38 weeks. Compared to HSUM, the Butterfly PUM had a mean difference of -0.20 days (95%CI±0.40) in the 1st trimester and -0.68 days (95%CI±0.68) in the 2nd/3rd trimesters. Also compared to HSUM, the Clarius PUM had a mean difference of 0.47 days (95%CI±0.64) in the 1st trimester and -1.67 days (95%CI±0.43) in the 2nd/3rd trimesters. ICCs were 0.989 or greater throughout. Increasing gestational age was associated with increasing error and absolute error. Both PUM devices demonstrated a modest trend toward underestimation of EGA at advancing gestational ages in 2nd/3rd trimester scans, compared to HSUM. CONCLUSION: Both the Butterfly iQ and Clarius C3 PUM devices were highly accurate in performing fetal biometry in a diverse population from the US and Zambia. This article is protected by copyright. All rights reserved.

Examination of the ovotoxicity of 5-fluorouracil in mice.

PURPOSE: Undesirable side effects of cancer treatments are common and include damage to the ovary, and depletion of the follicle reserve, which if severe enough, can lead to infertility and early menopause. Antimetabolite drugs, such as 5-fluorouracil (5-FU), are not considered to be detrimental to the ovary, but the ovotoxicity of 5-FU has not been evaluated in any detail. The purpose of this study was to evaluate the effects of 5-FU on follicle number. METHODS: In this study, adult female C57Bl6 mice (n = 4-6 animals/group) received a single dose of saline or 5-FU (150 mg/kg) and markers of ovarian damage and follicle depletion were assessed 12 h and 7 days later. RESULTS: Exposure to 5-FU did not alter primordial and primary follicle numbers. Atresia of secondary and antral follicles was increased significantly 12 h after 5-FU treatment, but atresia rates returned to levels similar to that of saline treated controls at 7 days. The number of corpora lutea were reduced 7 days after exposure to 5-FU, possibly as a consequence of earlier follicular atresia. CONCLUSIONS: These findings suggest that a single dose of 5-FU is mildly ovotoxic, but any effects on ovarian function are likely transient because the primordial follicle population is not depleted. Collectively, these data support the notion that 5-FU is unlikely to impact on the long-term fertility of women.

Dynamically controlled deposition of colloidal nanoparticle suspension in evaporating drops using laser radiation.

Dynamic control of the distribution of polystyrene suspended nanoparticles in evaporating droplets is investigated using a 2.9 µm high power laser. Under laser radiation a droplet is locally heated and fluid flows are induced that overcome the capillary flow, and thus a reversal of the coffee-stain effect is observed. Suspension particles are accumulated in a localised area, one order of magnitude smaller than the original droplet size. By scanning the laser beam over the droplet, particles can be deposited in an arbitrary pattern. This finding raises the possibility for direct laser writing of suspended particles through a liquid layer. Furthermore, a highly uniform coating is possible by manipulating the laser beam diameter and exposure time. The effect is expected to be universally applicable to aqueous solutions independent of solutes (either particles or molecules) and deposited substrates.

Post-natal imprinting: evidence from marsupials.

Genomic imprinting has been identified in therian (eutherian and marsupial) mammals but not in prototherian (monotreme) mammals. Imprinting has an important role in optimising pre-natal nutrition and growth, and most imprinted genes are expressed and imprinted in the placenta and developing fetus. In marsupials, however, the placental attachment is short-lived, and most growth and development occurs post-natally, supported by a changing milk composition tailor-made for each stage of development. Therefore there is a much greater demand on marsupial females during post-natal lactation than during pre-natal placentation, so there may be greater selection for genomic imprinting in the mammary gland than in the short-lived placenta. Recent studies in the tammar wallaby confirm the presence of genomic imprinting in nutrient-regulatory genes in the adult mammary gland. This suggests that imprinting may influence infant post-natal growth via the mammary gland as it does pre-natally via the placenta. Similarly, an increasing number of imprinted genes have been implicated in regulating feeding and nurturing behaviour in both the adult and the developing neonate/offspring in mice. Together these studies provide evidence that genomic imprinting is critical for regulating growth and subsequently the survival of offspring not only pre-natally but also post-natally.

Nurses' practice patterns in relation to adherence-enhancing interventions in stem cell transplant care: a survey from the Nurses Group of the European Group for Blood and Marrow Transplantation.

Recipients of stem cell transplants (SCT) must accurately manage multiple medications as non-adherence jeopardises treatment benefits. There is an evidence base for the efficacy of adherence-enhancing interventions; however, level of clinical implementation is unknown. This study aimed to identify patterns of practice in assessing medication adherence, screening for risk factors of non-adherence, interventions used in SCT to improve adherence and how nurses perceive the effectiveness of such interventions. A convenience sample of 143 European nurses completed a 29-item questionnaire measuring the frequency and perceived effectiveness of assessment/screening methods for adherence and three types of intervention (educational/cognitive, counselling/behavioural and psychological/affective). Questioning patients about adherence was the most regularly used assessment method (51.5%). Nurses used a median of seven interventions (interquartile range: six) 'frequently', the most popular being provision of reading materials (79%). The interventions perceived as most effective were; providing individual patient/family with teaching and reading materials. This is the first study exploring patterns of practice relating to adherence in SCT. Educational interventions were the most frequently employed style of intervention, which is at odds with recent data suggesting limited efficacy with this style of intervention. Combining educational, behavioural and psychological interventions would more accurately embrace current understanding.

Understanding Academic Self-Concept and Performance in Medical Education.

PROBLEM: Medical students' academic self-concept (ASC) is an important factor in better understanding noncognitive mediators of performance in medical school. However, research is limited on ASC in medical students across multiple phases of undergraduate medical education curriculum. This pilot study explored the relationship between ASC and academic performance across different phases of a U.S. medical school curriculum, specifically at the end of the second (preclinical) and third (clinical) years. APPROACH: Medical students across 2 cohorts at Virginia Commonwealth University School of Medicine, Richmond, Virginia, were surveyed using an ASC confidence subscale in 2019. Multiple linear regression analysis was conducted using medical student ASC scores in preclinical (n = 190) and clinical (n = 149) phases and performance data. Clinical performance was calculated through a weighted mean of clerkship grades based on the number of weeks for each clerkship. OUTCOMES: Preclinical performance was related to ASC, gender, and performance after year 1. ASC scores varied significantly by gender in the preclinical cohort ( P < .01), with men reporting higher ASC than women (mean [SD], 2.94 [0.41] vs 2.78 [0.38]). Significant gender differences in performance were found at the end of year 3 ( P < .01), with women performing more favorably compared with men (mean [SD], 94.1 [59.04] vs 124.24 [64.54]). The relationship between ASC and performance at the end of year 2 suggested students with higher ASC perform better during their preclinical phase. NEXT STEPS: This pilot study supports future scholarship in 2 areas: (1) identification and assessment of additional factors that influence the relationship between ASC and academic performance across the entire undergraduate medical education curriculum and (2) development and implementation of evidence-based interventions to support student ASC and performance and enhance the learning environment. Analyzing longitudinal trends across multiple cohorts will drive evidence-based interventions at learner and programmatic levels.

Measuring the Master Adaptive Learner: Development and Internal Structure Validity Evidence for a New Instrument.

BACKGROUND: The master adaptive learner (MAL) uses self-regulated learning skills to develop adaptive, efficient, and accurate skills in practice. Given rapid changes in healthcare, it is essential that medical students develop into MALs. There is a need for an instrument that can capture MAL behaviors and characteristics. The objective of this study was to develop an instrument for measuring the MAL process in medical students and evaluate its psychometric properties. METHODS: As part of curriculum evaluation, 818 students completed previously developed instruments with validity evidence including the Self-Regulated Learning Perception Scale, Brief Resilience Scale, Goal Orientation Scale, and Jefferson Scale of Physician Lifelong Learning. The authors performed exploratory factor analysis to examine underlying relationships between items. Items with high factor loadings were retained. Cronbach's alpha was computed. In parallel, the multi-institutional research team rated the same items to provide content validity evidence of the items to MAL model. RESULTS: The original 67 items were reduced to 28 items loading onto four factors: Planning, Learning, Resilience, and Motivation. Each subscale included the following number of items and Cronbach's alpha: Planning (10 items, alpha = 0.88), Learning (6 items, alpha = 0.81), Resilience (6 items, alpha = 0.89), and Motivation (6 items, alpha = 0.81). The findings from the factor analyses aligned with the research team ratings of linkage to the components of MAL. CONCLUSION: These findings serve as a starting point for future work measuring master adaptive learning to identify and support learners. To fully measure the MAL construct, additional items may need to be developed.

CD4+ cell count and risk for antiretroviral drug resistance among women using peripartum nevirapine for perinatal HIV prevention.

OBJECTIVE: To determine the association between the antenatal CD4(+) cell count and the development of viral drug resistance following the use of peripartum nevirapine (NVP) for perinatal HIV prevention. DESIGN: Secondary analysis of data from a previously conducted randomised controlled trial. SETTING: Lusaka, Zambia. POPULATION: HIV-positive pregnant women. METHODS: We analysed the data from a clinical trial of single-dose tenofovir/emtricitabine (TDF/FTC) to reduce viral drug resistance associated with peripartum NVP. The trial population was categorised according to antenatal CD4(+) cell count (200-350, 351-500 and >500 cells/µl). MAIN OUTCOME MEASURES: The relative risk for acquiring drug resistance, determined by consensus sequencing and oligonucleotide ligation assay (OLA), was estimated using multivariable logistic regression. RESULTS: Of the 397 study participants, 119 (30%) had a CD4(+) count of 200-350 cells/µl, 135 (34%) had a CD4(+) count of 351-500 cells/µl and 143 (36%) had a CD4(+) count of >500 cells/µl. Among women receiving no intervention, the risk for drug resistance appeared to increase as the CD4(+) cell count decreased. Participants with CD4(+) cell counts of 200-350 cells/µl randomised to the study arm had the lowest risk, suggesting a higher efficacy of the intervention within this stratum. These results were consistent at 2 and 6 weeks, regardless of how drug resistance was measured. CONCLUSIONS: Women with CD4(+) cell counts of 200-350 cells/µl may be at increased risk for viral drug resistance following the use of peripartum NVP. Given the high prevalence of NVP resistance and the clear benefits of treatment, antiretroviral therapy should be initiated among pregnant women with CD4(+) cell counts of ≤350 cells/µl.

Generalizability of the Ottawa Surgical Competency Operating Room Evaluation (O-SCORE) Scale to Assess Medical Student Performance on Core EPAs in the Workplace: Findings From One Institution.

PURPOSE: Assessment of the Core Entrustable Professional Activities for Entering Residency (Core EPAs) requires direct observation of learners in the workplace to support entrustment decisions. The purpose of this study was to examine the internal structure validity evidence of the Ottawa Surgical Competency Operating Room Evaluation (O-SCORE) scale when used to assess medical student performance in the Core EPAs across clinical clerkships. METHOD: During the 2018-2019 academic year, the Virginia Commonwealth University School of Medicine implemented a mobile-friendly, student-initiated workplace-based assessment (WBA) system to provide formative feedback for the Core EPAs across all clinical clerkships. Students were required to request a specified number of Core EPA assessments in each clerkship. A modified O-SCORE scale (1 = "I had to do" to 4 = "I needed to be in room just in case") was used to rate learner performance. Generalizability theory was applied to assess the generalizability (or reliability) of the assessments. Decision studies were then conducted to determine the number of assessments needed to achieve a reasonable reliability. RESULTS: A total of 10,680 WBAs were completed on 220 medical students. The majority of ratings were completed on EPA 1 (history and physical) (n = 3,129; 29%) and EPA 6 (oral presentation) (n = 2,830; 26%). Mean scores were similar (3.5-3.6 out of 4) across EPAs. Variance due to the student ranged from 3.5% to 8%, with the majority of the variation due to the rater (29.6%-50.3%) and other unexplained factors. A range of 25 to 63 assessments were required to achieve reasonable reliability (Phi > 0.70). CONCLUSIONS: The O-SCORE demonstrated modest reliability when used across clerkships. These findings highlight specific challenges for implementing WBAs for the Core EPAs including the process for requesting WBAs, rater training, and application of the O-SCORE scale in medical student assessment.

Examining Bloom's Taxonomy in Multiple Choice Questions: Students' Approach to Questions.

BACKGROUND: Analytic thinking skills are important to the development of physicians. Therefore, educators and licensing boards utilize multiple-choice questions (MCQs) to assess these knowledge and skills. MCQs are written under two assumptions: that they can be written as higher or lower order according to Bloom's taxonomy, and students will perceive questions to be the same taxonomical level as intended. This study seeks to understand the students' approach to questions by analyzing differences in students' perception of the Bloom's level of MCQs in relation to their knowledge and confidence. METHODS: A total of 137 students responded to practice endocrine MCQs. Participants indicated the answer to the question, their interpretation of it as higher or lower order, and the degree of confidence in their response to the question. RESULTS: Although there was no significant association between students' average performance on the content and their question classification (higher or lower), individual students who were less confident in their answer were more than five times as likely (OR = 5.49) to identify a question as higher order than their more confident peers. Students who responded incorrectly to the MCQ were 4 times as likely to identify a question as higher order than their peers who responded correctly. CONCLUSIONS: The results suggest that higher performing, more confident students rely on identifying patterns (even if the question was intended to be higher order). In contrast, less confident students engage in higher-order, analytic thinking even if the question is intended to be lower order. Better understanding of the processes through which students interpret MCQs will help us to better understand the development of clinical reasoning skills.

Nevirapine-associated hepatotoxicity was not predicted by CD4 count ≥250 cells/µL among women in Zambia, Thailand and Kenya.

OBJECTIVE: The aim of the study was to determine risk factors for developing severe hepatotoxicity (grade 3 or 4 hepatotoxicity) and rash-associated hepatotoxicity (rash with ≥ grade 2 hepatotoxicity) among women initiating nevirapine-based antiretroviral therapy (ART). METHODS: The Non-Nucleoside Reverse Transcriptase Inhibitor Response Study was a prospective cohort study carried out in Zambia, Thailand and Kenya. Between May 2005 and January 2007, we enrolled antiretroviral-naïve HIV-infected women initiating nevirapine-based ART. At enrollment and at weeks 2, 4, 8, 16 and 24, participants had serum alanine transferase (ALT) and aspartate transaminase (AST) measured and were evaluated clinically for hepatitis and rash. RESULTS: Nevirapine-based ART was initiated in 820 women and baseline ALT or AST results were abnormal (≥ grade 1) in 113 (14%) women. After initiating nevirapine-based ART, severe hepatotoxicity occurred in 41 (5%) women and rash-associated hepatotoxicity occurred in 27 (3%) women. In a multivariate logistic regression model, severe hepatotoxicity and rash-associated hepatotoxicity were both associated with baseline abnormal (≥ grade 1) ALT or AST results, but not with a baseline CD4 cell count ≥250 cells/µL. Three participants (0.4%) died with symptoms suggestive of fatal hepatotoxicity; all three women had baseline CD4 count <100 cells/µL and were receiving anti-tuberculosis therapy. CONCLUSION: Among women taking nevirapine-based ART, severe hepatotoxicity and rash-associated hepatotoxicity were predicted by abnormal baseline ALT or AST results, but not by a CD4 count ≥250 cells/µL. In resource-limited settings where transaminase testing is available, testing should focus on early time-points and on women with abnormal baseline ALT or AST results.

Hematopoietic cell transplant nurse coordinators' perceptions of related donor care: a European survey from the EBMT Nurses Group.

Allogeneic haematopoietic cell transplantation (HCT) is a curative procedure for patients with haematological malignancies and immune deficiencies. A human leukocyte antigen (HLA) identical sibling is only available for 25-35% of patients in need. The improvement in haplo-identical transplantation has led to a marked increase in cell donation from relatives. Despite international recommendations, discrepancies in related-donors (RD) care exist between centres, particularly regarding medical suitability criteria, consenting procedures and donor follow-up. This European survey aimed to explore hematopoietic cell transplantation coordinators nurses' (HCT-CNs) perceptions of RD care, in particular the association with the presence or not of an independent unit (IU). Ninety-three HCT-CNs from seventy-six EBMT centres responded, representing 19 countries (response rate: 27%). Our results did not show a significant association between IU and HCT-CNs perceptions of related-donors care. The practices for RD care vary among centres regarding presence or not of an IU (48%), person caring for RD (haematologist in 54%, HCT physician in 17%, HCT-CNs in 20%), person to whom the results of HLA typing are communicated, use of a booklet for RD, follow-up or not and periodicity of follow-up. Qualitative data highlight the related-donation ethical issues and the need for improvement in RD care. HCT-CNs' main concerns were: the necessary confidentiality to insure the voluntary status of RD, the perceived conflict of interest felt by professionals when managing both patients and RD, plus the psychosocial aspects of related-donation. Even if there is a variety of a practice among centres, the presence of an IU is not significantly associated with an improvement in RD care.

A family of cation ATPase-like molecules from Plasmodium falciparum.

We report the nucleotide and derived amino acid sequence of the ATPase 1 gene from Plasmodium falciparum. The amino acid sequence shares homology with the family of "P"-type cation translocating ATPases in conserved regions important for nucleotide binding, conformational change, or phosphorylation. The gene, which is present on chromosome 5, has a product longer than any other reported for a P-type ATPase. Interstrain analysis from 12 parasite isolates by the polymerase chain reaction reveals that a 330-bp nucleotide sequence encoding three cytoplasmic regions conserved in cation ATPases (regions a-c) is of constant length. By contrast, another 360-bp sequence which is one of four regions we refer to as "inserts" contains arrays of tandem repeats which show length variation between different parasite isolates. Polymorphism results from differences in the number and types of repeat motif contained in this insert. Inserts are divergent in sequence from other P-type ATPases and share features in common with many malarial antigens. Studies using RNA from the erythrocytic stages of the malarial life cycle suggest that ATPase 1 (including the sequence which encodes tandem repeats) is expressed at the large ring stage of development. Immunolocalization has identified ATPase 1 to be in the region of the parasite plasma membrane and pigment body. These findings suggest a possible model for the genesis of malarial antigens.

Proximity of chromosomal loci that participate in radiation-induced rearrangements in human cells.

Rearrangements involving the RET gene are common in radiation-associated papillary thyroid cancer (PTC). The RET/PTC1 type of rearrangement is an inversion of chromosome 10 mediated by illegitimate recombination between the RET and the H4 genes, which are 30 megabases apart. Here we ask whether despite the great linear distance between them, RET and H4 recombination might be promoted by their proximity in the nucleus. We used two-color fluorescence in situ hybridization and three-dimensional microscopy to map the positions of the RET and H4 loci within interphase nuclei. At least one pair of RET and H4 was juxtaposed in 35% of normal human thyroid cells and in 21% of peripheral blood lymphocytes, but only in 6% of normal mammary epithelial cells. Spatial contiguity of RET and H4 may provide a structural basis for generation of RET/PTC1 rearrangement by allowing a single radiation track to produce a double-strand break in each gene at the same site in the nucleus.

Community-based follow-up for late patients enrolled in a district-wide programme for antiretroviral therapy in Lusaka, Zambia.

Timely adherence to clinical and pharmacy appointments is well correlated with favourable patient outcomes among HIV-infected individuals on antiretroviral therapy. To date, however, there is little work exploring reasons behind missed visits or evaluating programmatic strategies to recall patients. For this study we implemented community-based follow-up of late patients as part of a large-scale programme for HIV care and treatment in Lusaka, Zambia. Through a network of local home-based care organizations, we attempted home visits to recall patients using locator information provided at time of enrolment. Between May and September 2005, home-based caregivers were dispatched to trace 1,343 patients with missed appointments. Of these, 554 (41%) were untraceable because the provided address was invalid, the patient had moved or no one was at the home. Of the remaining 789, 359 (46%) were reported to have died. Only 430 (54% of those traced, 32% overall) were contacted directly and encouraged to return for care. The likelihood of patient return was higher among traced patients in crude analysis (relative risk [RR] = 2.5; 95%CI = 1.9-3.2) and in multivariable analysis controlling for baseline body mass index, sex and CD4 + count < or = 50/microL (adjusted RR = 2.3; 95%CI = 1.7-3.2). However, the process was inefficient: one late patient returned for every 18 home visits that were made. Reasons for missed visits were provided in 271 of 430 (63%) of the patients who were successfully traced. Common reasons included feeling too sick to come to the clinic, travelling away from home and being too busy. Despite the availability of free ART in Lusaka, patients face significant barriers to attending scheduled clinical visits. Cost-effective and feasible strategies are urgently needed to improve timely patient follow-up.

Leber Hereditary Optic Neuropathy and Longitudinally Extensive Transverse Myelitis.

Leber Hereditary Optic Neuropathy is an inherited optic neuropathy caused by mitochondrial DNA point mutations leading to sudden, painless loss of vision. We report a case of an 8-year-old boy presenting with a radiological phenotype of longitudinally extensive transverse myelitis on a background of severe visual impairment secondary to Leber Hereditary Optic Neuropathy (LHON). He was found to have dual mitochondrial DNA mutations at 14484 (MTND6 gene) and 4160 (MTND1 gene) in a family with a severe form of LHON characterised by not only an unusually high penetrance of optic neuropathy, but also severe extra-ocular neurological complications. The m.14484T>C mutation is a common LHON mutation, but the m.4160T>C mutation is to our knowledge not reported outside this family and appears to drive the neurological manifestations. To our knowledge there have been no previous reports of spinal cord lesions in children with LHON.

Interphase chromosome folding determines spatial proximity of genes participating in carcinogenic RET/PTC rearrangements.

Recurrent chromosomal rearrangements are common in cancer cells and may be influenced by nonrandom positioning of recombination-prone genetic loci in the nucleus. However, the mechanism responsible for spatial proximity of specific loci is unknown. In this study, we use an 18 Mb region on 10q11.2-21 containing the RET gene and its recombination partners, the H4 and NCOA4 (ELE1) genes, as a model chromosomal region frequently involved in RET/PTC rearrangements in thyroid cancer. RET/PTC is particularly common in tumors from children exposed to ionizing radiation. Using fluorescence in situ hybridization and three-dimensional microscopy, the locations of five different loci in this region were mapped in interphase nuclei of normal human thyroid cells. We show that RET and NCOA4 are much closer to each other than expected based on their genomic separation. Modeling of chromosome folding in this region suggests the presence of chromosome coiling with coils of approximately 8 Mb in length, which positions the RET gene close to both, the NCOA4 and H4, loci. There was no significant variation in gene proximity between adult and pediatric thyroid cells. This study provides evidence for large-scale chromosome folding of the 10q11.2-21 region that offers a structural basis for nonrandom positioning and spatial proximity of potentially recombinogenic intrachromosomal loci.

Recombination between poly[d(GT).d(CA)] sequences in simian virus 40-infected cultured cells.

CVI cells were transfected with oversized simian virus 40 (SV40) genomes that could be reduced to packageable size by alternative homologous recombination pathways involving either two polydeoxyguanylic-thymidylic acid X polydeoxycytidylic-adenylic acid (poly[d(GT).d(CA)]; abbreviated hereafter as poly(GT)] tracts or two tracts of homologous SV40 sequence. Plaque-forming viruses rescued by this procedure were found to contain genomes formed by homologous and nonhomologous recombination events. Half of the viable viral DNA molecules recovered were the result of recombination between two tracts of poly(GT). Approximately 20% of the rescued viral genomes were produced by homologous recombination between tracts of SV40 DNA. Nonhomologous recombination involving SV40 sequences was also a major pathway of deletion, producing ca. 30% of the viral plaques. Tracts of poly(GT) generated by recombination were variable in length, suggesting that recombination between poly(GT) tracts was usually unequal. On a per-nucleotide basis, poly(GT) recombination occurred eight times more frequently than did recombination between homologous SV40 DNA. This eightfold difference is the maximum recombinatory enhancement attributable to poly(GT) sequences. Although DNA sequence analysis showed that tracts of poly(GT) generated by recombination retained the alternating G-T repeat motif throughout their length, the contribution of the nonhomologous pathway to poly(GT) recombination cannot be ruled out, and the relative proclivity of a given length of d(GT).d(CA) sequence to undergo homologous recombination is probably less than eight times greater than that of an SV40 sequence of the same length.

Structure and expression of a tandem gene pair in Leishmania donovani that encodes a protein structurally homologous to eucaryotic cation-transporting ATPases.

An oligonucleotide probe was used to clone a cation-transporting ATPase gene from the genome of Leishmania donovani. The nucleotide sequence of the gene contained a 2,922-base-pair open reading frame that was predicted to encode a 107,406-dalton protein composed of 974 amino acids. The predicted L. donovani protein contained all the structural and functional domains expected to be present in a cation-transporting ATPase of the aspartyl phosphate class. The nucleotide sequence encoding the ATPase gene was duplicated in tandem in the parasite genome. Partial sequenation of the second member of the tandem repeat, which lay 2 kilobase pairs downstream of the ATPase gene, indicated that it was either identical to the first gene or very closely related to it. RNA homologous to either the ATPase gene or its adjacent relative was 5 kilobases in size and was approximately equally abundant in both promastigote and amastigote forms of the organism.

Unequal homologous recombination between tandemly arranged sequences stably incorporated into cultured rat cells.

Cultured rat cells deficient in endogenous thymidine kinase activity (tk) were stably transformed with a recombination-indicator DNA substrate constructed in vitro by rearrangement of the herpes simplex virus tk gene sequences into a partially redundant permutation of the functional gene. The recombination-indicator DNA did not express tk, but was designed to allow formation of a functional tk gene via homologous recombination. A clonal cell line (519) was isolated that harbored several permuted herpes simplex virus tk genes. 519 cells spontaneously produced progeny that survived in medium containing hypoxanthine, aminopterin, and thymidine. Acquisition of resistance to hypoxanthine, aminopterin, and thymidine was accompanied by the rearrangement of the defective tk gene to functional configuration. The rearrangement apparently occurred by unequal exchange between one permuted tk gene and a replicated copy of itself. Recombination was between 500-base-pair tracts of DNA sequence homology that were separated by 3.4 kilobases. Exchanges occurred spontaneously at a frequency of approximately 5 X 10(-6) events per cell per generation. Recombination also mediated reversion to the tk- phenotype; however, the predominant mechanism by which cells escaped death in the presence of drugs rendered toxic by thymidine kinase was not recombination, but rather inactivation of the intact tk gene.