Interactive effects of CO2 , temperature, irradiance, and nutrient limitation on the growth and physiology of the marine cyanobacterium Synechococcus (Cyanophyceae).

The marine cyanobacterium Synechococcus elongatus was grown in a continuous culture system to study the interactive effects of temperature, irradiance, nutrient limitation, and the partial pressure of CO2 (pCO2) on its growth and physiological characteristics. Cells were grown on a 14:10 h light:dark cycle at all combinations of low and high irradiance (50 and 300 µmol photons ⋅ m-2 ⋅ s-1 , respectively), low and high pCO2 (400 and 1000 ppmv, respectively), nutrient limitation (nitrate-limited and nutrient-replete conditions), and temperatures of 20-45°C in 5°C increments. The maximum growth rate was ~4.5 · d-1 at 30-35°C. Under nutrient-replete conditions, growth rates at most temperatures and irradiances were about 8% slower at a pCO2 of 1000 ppmv versus 400 ppmv. The single exception was 45°C and high irradiance. Under those conditions, growth rates were ~45% higher at 1000 ppmv. Cellular carbon:nitrogen ratios were independent of temperature at a fixed relative growth rate but higher at high irradiance than at low irradiance. Initial slopes of photosynthesis-irradiance curves were higher at all temperatures under nutrient-replete versus nitrate-limited conditions; they were similar at all temperatures under high and low irradiance, except at 20°C, when they were suppressed at high irradiance. A model of phytoplankton growth in which cellular carbon was allocated to structure, storage, or the light or dark reactions of photosynthesis accounted for the general patterns of cell composition and growth rate. Allocation of carbon to the light reactions of photosynthesis was consistently higher at low versus high light and under nutrient-replete versus nitrate-limited conditions.

Interactive Effects of CO2 , Temperature, Irradiance, and Nutrient Limitation on the Growth and Physiology of the Marine Diatom Thalassiosira pseudonana (Coscinodiscophyceae).

The marine diatom Thalassiosira pseudonana was grown in continuous culture systems to study the interactive effects of temperature, irradiance, nutrient limitation, and the partial pressure of CO2 (pCO2 ) on its growth and physiological characteristics. The cells were able to grow at all combinations of low and high irradiance (50 and 300 µmol photons · m-2  · s-1 , respectively, of visible light), low and high pCO2 (400 and 1,000 µatm, respectively), nutrient limitation (nitrate-limited and nutrient-replete conditions), and temperatures of 10-32°C. Under nutrient-replete conditions, there was no adverse effect of high pCO2 on growth rates at temperatures of 10-25°C. The response of the cells to high pCO2 was similar at low and high irradiance. At supraoptimal temperatures of 30°C or higher, high pCO2 depressed growth rates at both low and high irradiance. Under nitrate-limited conditions, cells were grown at 38 ± 2.4% of their nutrient-saturated rates at the same temperature, irradiance, and pCO2 . Dark respiration rates consistently removed a higher percentage of production under nitrate-limited versus nutrient-replete conditions. The percentages of production lost to dark respiration were positively correlated with temperature under nitrate-limited conditions, but there was no analogous correlation under nutrient-replete conditions. The results suggest that warmer temperatures and associated more intense thermal stratification of ocean surface waters could lower net photosynthetic rates if the stratification leads to a reduction in the relative growth rates of marine phytoplankton, and at truly supraoptimal temperatures there would likely be a synergistic interaction between the stresses from temperature and high pCO2 (lower pH).

Proteomic and pathway analyses reveal a network of inflammatory genes associated with differences in skin tumor promotion susceptibility in DBA/2 and C57BL/6 mice.

Genetic susceptibility to two-stage skin carcinogenesis is known to vary significantly among different stocks and strains of mice. In an effort to identify specific protein changes or altered signaling pathways associated with skin tumor promotion susceptibility, a proteomic approach was used to examine and identify proteins that were differentially expressed in epidermis between promotion-sensitive DBA/2 and promotion-resistant C57BL/6 mice following treatment with 12-O-tetradecanoylphorbol-13-acetate (TPA). We identified 19 differentially expressed proteins of which 5 were the calcium-binding proteins annexin A1, parvalbumin α, S100A8, S100A9, and S100A11. Further analyses revealed that S100A8 and S100A9 protein levels were also similarly differentially upregulated in epidermis of DBA/2 versus C57BL/6 mice following topical treatment with two other skin tumor promoters, okadaic acid and chrysarobin. Pathway analysis of all 19 identified proteins from the present study suggested that these proteins were components of several networks that included inflammation-associated proteins known to be involved in skin tumor promotion (e.g. TNF-α, NFκB). Follow-up studies revealed that Tnf, Nfkb1, Il22, Il1b, Cxcl1, Cxcl2 and Cxcl5 mRNAs were highly expressed in epidermis of DBA/2 compared with C57BL/6 mice at 24h following treatment with TPA. Furthermore, NFκB (p65) was also highly activated at the same time point (as measured by phosphorylation at ser276) in epidermis of DBA/2 mice compared with C57BL/6 mice. Taken together, the present data suggest that differential expression of genes involved in inflammatory pathways in epidermis may play a key role in genetic differences in susceptibility to skin tumor promotion in DBA/2 and C57BL/6 mice.

Protection against 2-chloroethyl ethyl sulfide (CEES)-induced cytotoxicity in human keratinocytes by an inducer of the glutathione detoxification pathway.

Sulfur mustard (SM or mustard gas) was first used as a chemical warfare agent almost 100years ago. Due to its toxic effects on the eyes, lungs, and skin, and the relative ease with which it may be synthesized, mustard gas remains a potential chemical threat to the present day. SM exposed skin develops fluid filled bullae resulting from potent cytotoxicity of cells lining the basement membrane of the epidermis. Currently, there are no antidotes for SM exposure; therefore, chemopreventive measures for first responders following an SM attack are needed. Glutathione (GSH) is known to have a protective effect against SM toxicity, and detoxification of SM is believed to occur, in part, via GSH conjugation. Therefore, we screened 6 potential chemopreventive agents for ability to induce GSH synthesis and protect cultured human keratinocytes against the SM analog, 2-chloroethyl ethyl sulfide (CEES). Using NCTC2544 human keratinocytes, we found that both sulforaphane and methyl-2-cyano-3,12-dioxooleana-1,9-dien-28-oate (CDDO-Me) stimulated nuclear localization of Nrf2 and induced expression of the GSH synthesis gene, GCLM. Additionally, we found that treatment with CDDO-Me elevated reduced GSH content of NCTC2544 cells and preserved their viability by ~3-fold following exposure to CEES. Our data also suggested that CDDO-Me may act additively with 2,6-dithiopurine (DTP), a nucleophilic scavenging agent, to increase the viability of keratinocytes exposed to CEES. These results suggest that CDDO-Me is a promising chemopreventive agent for SM toxicity in the skin.

In vivo transfer of GPI-linked complement restriction factors from erythrocytes to the endothelium.

Many proteins are associated with the outer layer of the cell membrane through a posttranslationally added glycosyl phosphatidylinositol (GPI) anchor. The functional significance of this type of protein linkage is unclear, although it results in increased lateral mobility, sorting to the apical surface of the cell, reinsertion into cell membranes, and possibly cell signaling. Here evidence is presented that GPI-linked proteins can undergo intermembrane transfer in vivo. GPI-linked proteins expressed on the surface of transgenic mouse red blood cells were transferred in a functional form to endothelial cells in vivo. This feature of GPI linkage may be potentially useful for the delivery of therapeutic proteins to vascular endothelium.

MIP-1alpha regulates CD4+ T cell chemotaxis and indirectly enhances PMN persistence in Pseudomonas aeruginosa corneal infection.

The role of macrophage inflammatory protein-1alpha (MIP-1alpha) in cell infiltration into Pseudomonas aeruginosa-infected cornea and subsequent disease was examined. Greater amounts of the chemokine (protein and mRNA) were found in the infected cornea of susceptible B6 ("cornea perforates") versus resistant BALB/c ("cornea heals") mice from 1 to 5 days postinfection. Treatment of BALB/c mice with recombinant (r) MIP-1alpha exacerbated disease and was associated with an increased number of neutrophils (PMNs) in the cornea. Treatment of BALB/c mice with rMIP-1alpha also induced recruitment of activated CD4+ T cells into the affected cornea, converting resistant to susceptible mice. Depleting CD4+ T cells in r-treated BALB/c mice significantly decreased PMNs in cornea tissue, suggesting that T cells regulate persistence of PMNs at this site. In B6 mice, administration of neutralizing MIP-1alpha polyclonal antibody also significantly reduced PMN numbers and pathology. Collectively, evidence is provided that MIP-1alpha directly contributed to CD4+ T cell recruitment and indirectly to PMN persistence in the infected cornea.

ErbB2-intronic microRNA-4728: a novel tumor suppressor and antagonist of oncogenic MAPK signaling.

Although the role of the ErbB2/HER2 oncogene in cancers has been extensively studied, how ErbB2 is regulated remains poorly understood. A novel microRNA, mir-4728, was recently found within an intron of the ErbB2 gene. However, the function and clinical relevance of this intronic miRNA are completely unknown. Here, we demonstrate that mir-4728 is a negative regulator of MAPK signaling through directly targeting the ERK upstream kinase MST4 and exerts numerous tumor-suppressive properties in vitro and in animal models. Importantly, our patient sample study shows that mir-4728 was under-expressed in breast tumors compared with normal tissue, and loss of mir-4728 correlated with worse overall patient survival. These results strongly suggest that mir-4728 is a tumor-suppressive miRNA that controls MAPK signaling through targeting MST4, revealing mir-4728's significance as a potential prognostic factor and target for therapeutic intervention in cancer. Moreover, this study represents a conceptual advance by providing strong evidence that a tumor-suppressive miRNA can antagonize the canonical signaling of its host oncogene.

Severe bleeding diathesis associated with invasive aspergillosis in transplant patients.

A severe bleeding disorder developed in eight renal transplant patients with invasive aspergillosis. The hemorrhagic diathesis was characterized by wound oozing, severe upper and lower gastrointestinal tract hemorrhage, and mucosal bleeding at other sites. This unusual coagulopathy was characterized by a prolonged thrombin time, which was corrected with protamine sulfate, and an abnormal Reptilase time. The bleeding disorder antedated the diagnosis of invasive aspergillosis in all cases. The probability that the coagulopathy was due to proteolytic enzymes elaborated by Aspergillus sp. is discussed.

Identification and characterization of a galactosyl peptide mimetic. Implications for use in removing xenoreactive anti-A Gal antibodies.

Gal alpha 1,3 Gal is thought to be the major antigenic epitope present on pig tissues to which XNAs bind. Removal of antibodies directed against that structure may be critical to the success of pig to human xeno-transplantation. As a first step toward the development of ligands capable of removing XNAs, we have used a phage-displayed peptide library to identify a six-amino-acid peptide that binds to the lectin GS-1-B4 (which binds the carbohydrate Gal alpha 1,3 Gal). This peptide blocks the binding of GS-1-B4 to pig aortic endothelial cells. The carbohydrate Gal alpha 1,3 Gal competes with the binding of GS-1-B4 to the peptide, suggesting that they may bind the same site. Using a RBC agglutination assay, we show that this peptide inhibits the agglutination of pig RBCs by heat-inactivated human serum at concentrations similar to that of Gal alpha 1,3 Gal.

Transgenic pigs expressing human CD59 and decay-accelerating factor produce an intrinsic barrier to complement-mediated damage.

We characterize a line of transgenic pigs that express the human complement-regulatory proteins human CD59 and human decay-accelerating factor. These genes, under the control of heterologous promoters, are expressed in a variety of organs, including the vasculature of the heart, kidney, and liver. We demonstrate that moderate levels of these gene products are sufficient to protect peripheral blood cells from human or baboon complement. Using pig to baboon heterotopic heart transplants, we show that expression of these proteins is sufficient to block the complement-mediated damage that is the hallmark of such xenografts, when nontransgenic organs are used. These results indicate that there is significant species specificity of intrinsic complement regulatory protein function. This specificity is evident in transgenic organs in which low levels of human CD59 and human decay-accelerating factor expression significantly effect the humoral immune response that causes xenograft rejection. This result suggests that transgenic organs with high levels of human complement-regulatory protein expression will be sufficient to alleviate the humoral immunological barriers that currently block the use of xenogeneic organs for human transplantation.

Characterization of transgenic pigs expressing functionally active human CD59 on cardiac endothelium.

The critical shortage of human donor organs has generated interest in the potential for porcine to human xenotransplantation. The initial immunological barrier to xenotransplantation is hyperacute rejection, which is mediated by xenoreactive antibodies and complement, and results in rapid and irreversible tissue destruction. While endogenous complement regulatory proteins (CRPs) protect cells from injury caused by autologous complement, they are relatively species specific and most likely ineffectual in this setting. This has led to the hypothesis that expression of human CRPs in transgenic pigs may affect susceptibility to complement-mediated tissue injury in a porcine-to-human xenograft. Using specific lines of transgenic pigs that express low levels of human CD59, a CRP that acts at the terminal stage of the complement cascade, we present evidence that shows that the human CD59 protein inhibits membrane attack complex assembly and reduces tissue damage when the heart is transplanted to a baboon. Examination by immunohistochemistry of transgenic porcine hearts after transplantation revealed markedly reduced deposition of C5b and MAC, but a similar level of C3 deposition as compared with transplanted control hearts. This finding supports the concept that the species specific function of CRPs contributes to the humoral barrier to xenotransplantation and, given the low level of human CD59 protein expression in the porcine heart, argues that the human protein contributes a unique rather than an additive function in regulation of complement in a xenogeneic setting.

Increased severity of Pseudomonas aeruginosa corneal infection in strains of mice designated as Th1 versus Th2 responsive.

PURPOSE: Mice favoring Th1 (C57BL/6, C57BL/10, and B10.D2/nSn) versus Th2 (BALB/c, BALB/cBy, BALB.B, and BALB.K) response development were evaluated for their response to infection with Pseudomonas aeruginosa. This study addresses the question of whether Th1 versus Th2 response propensity affects the pathogenesis of bacterial keratitis in mice. METHODS: Ocular disease was determined by mean clinical score, slit lamp, plate counts, and histopathology, and antigen-specific cellular responses were assessed by immunostaining and measurement of delayed type hypersensitivity (DTH). RESULTS: Strains of mice favoring Th1 (B6, BL10, and B10.D2) versus Th2 (BALB/c, BALB/cBy, BALB.B, and BALB.K) responsiveness were infected with P. aeruginosa. Mice favoring Th1 response development exhibited a similar course of disease and the infected eyes of all mice perforated by 7 days postinfection (p.i.). Strains (BALB/c, BALB/cBy, BALB.B, and BALB.K) favoring Th2 response development exhibited a milder course of disease, and none of the infected corneas perforated at 7 days p.i. In a Th1-responsive strain (B10.D2), positive immunostaining for CD4+ and CD8+ T cells was observed in the cornea by 3 days p.i. and by 5 days p.i., respectively, some cells stained positively for IL2-R, indicating that the cells were activated. In contrast, in a Th2 responder strain (BALB/c), there was no detectable positive immunostaining in cornea for any of the T-cell markers tested and DTH was significantly elevated in B10.D2 versus BALB/c mice. CONCLUSIONS: These studies are the first to provide evidence that in P. aeruginosa ocular infection, mouse strains favoring development of a Th1-type response are susceptible (cornea perforates), whereas strains favoring Th2 response development are resistant (no corneal perforation).

Aging and PMN response to P. aeruginosa infection.

PURPOSE: Alterations in immune system function associated with aging may contribute to increased morbidity in this population of individuals. The current studies were performed to determine aging-related changes in polymorphonuclear neutrophil (PMN) function after corneal infection with Pseudomonas aeruginosa. METHODS: Total PMN number, macrophage inflammatory protein (MIP)-2 mRNA and protein expression, and ocular bacterial load were determined in 8-week- and 12-month-old inbred BALB/c mice at various times after infection with P. aeruginosa. In addition, 12-month-old mice were treated systemically with the MIP-2 polyclonal antibody (pAb) to determine the effects of MIP-2 neutralization on ocular disease and PMN recruitment. RESULTS: Histologically, PMN infiltration into the cornea of 12-month-old mice was delayed initially and was associated with an inability to reduce bacterial load at later postinfection (PI) times. In addition, a significantly greater number of PMNs were found in the cornea of 12-month-old mice at later PI times. The increase in PMN number in 12-month-old mice correlated with a persistence of MIP-2 expression in cornea at these later times. Systemic treatment of 12-month-old mice with neutralizing MIP-2 pAb versus normal rabbit serum (NRS) resulted in reduced corneal PMN number and ocular disease. CONCLUSIONS: These data provide evidence that persistence of PMN in the cornea of 12-month-old mice contributes to corneal tissue destruction after P. aeruginosa challenge. Further evidence also is provided that the chemoattractant MIP-2 contributes to the altered PMN response in these animals.

A prospective trial of telepathology for intraoperative consultation (frozen sections).

Telepathology is a maturing technology that, for a variety of reasons, has not been widely deployed. In addition, clinical validation is relatively modest compared with accepted telemedicine applications such as teleradiology. A prototype telepathology system (Tele-Path(sm)) featuring high-resolution images selected from a remote microscope site has been developed at the University of Alabama at Birmingham (UAB). To validate the diagnostic efficacy of the system, a prospective study was undertaken of parallel diagnoses by conventional microscopy and telepathology with a remotely operated microscope. Slides from 99 intraoperative consultations from 29 tissue/ organ sites in the University of Alabama Hospitals by 9 academic pathologists were used in the study. Each microscopic and telepathology diagnosis was compared with the final diagnosis rendered by a referee pathologist. Diagnoses were classified as correct, false positive, or false negative or classification error. Of the 99 frozen sections evaluated, 3 cases were deferred. Of the remaining 96 cases, 2 received incorrect diagnoses in both the microscopic and telepathology arms of the study. Three errors occurred only in the telepathology arm. There was 1 false-positive diagnosis, 1 false-negative diagnosis, and 1 classification error. Statistical analysis indicated no significant difference between telepathology and conventional microscopy. Qualitative data indicated that the pathologists were generally satisfied with the performance of the system. Telepathology using this system paradigm is sufficiently accurate for real time utilization in a complex surgical environment. Telepathology therefore may be an effective model to support the surgical services of hospitals lacking full-time pathology coverage, resulting in full-time access to anatomic pathology services.

Partial characterization of the atypical creatine kinase isoenzyme fraction in three cases.

The atypical creatine kinase isoenzyme fraction in the sera of three patients was studied utilizing the technic of crossed immunoelectrophoresis. The findings provide strong evidence that in these clinically unrelated patients the atpical isoenzyme fraction is a complex of creatine kinase and IgG.

An initial trial of a prototype telepathology system featuring static imaging with discrete control of the remote microscope.

Routine diagnosis of pathology images transmitted over telecommunications lines remains an elusive goal. Part of the resistance stems from the difficulty of enabling image selection by the remote pathologist. To address this problem, a telepathology microscope system (TelePath, TeleMedicine Solutions, Birmingham, Ala) that has features associated with static and dynamic imaging systems was constructed. Features of the system include near real time image transmission, provision of a tiled overview image, free choice of any fields at any desired optical magnification, and automated tracking of the pathologist's image selection. All commands and images are discrete, avoiding many inherent problems of full motion video and continuous remote control. A set of 64 slides was reviewed by 3 pathologists in a simulated frozen section environment. Each pathologist provided diagnoses for all 64 slides, as well as qualitative information about the system. Thirty-one of 192 diagnoses disagreed with the reference diagnosis that had been reached before the trial began. Qf the 31, 13 were deferrals and 12 were diagnoses of cases that had a deferral as the reference diagnosis. In 6 cases, the diagnosis disagreed with the reference diagnosis yielding an overall accuracy of 96.9%. Confidence levels in the diagnoses were high. This trial suggests that this system provides high-quality anatomic pathology services, including intraoperative diagnoses, over telecommunications lines.

Complement defects in aged mice compromise phagocytosis of Pseudomonas aeruginosa.

PURPOSE: The role of complement in phagocytosis and killing of P. aeruginosa was examined using serum from aged vs young donor mice. METHODS: Phagocytosis, complement hemolytic and microbicidal assays were used. RESULTS: Serum from young donor mice contained a heat-labile factor which significantly enhanced phagocytic activity of cells from young mice compared with similarly treated aged donor serum. Use of cobra venom factor (CVF) to destroy C3 and the terminal complement components in serum from young or aged donor mice also significantly decreased the phagocytic activity of young cells. EGTA treatment of young or aged donor serum, to activate the alternative pathway and selectively inhibit activation of the classical pathway, resulted in a significant decrease in phagocytosis by young cells in the presence of donor serum from either group. Alternative pathway mediated hemolysis also was measured and was significantly reduced in aged vs young donor serum. PMN microbicidal activity was tested using cells from young mice in the presence of aged vs young donor serum, but no significant differences were noted. CONCLUSION: These data provide evidence that defects in the alternative pathway of complement in the serum of aged animals lead to decreased phagocytic activity of cells from young mice, but not impaired bacterial killing.

Applications of virtual reality technology in pathology.

TelePath(SM) a telerobotic system utilizing virtual microscope concepts based on high quality still digital imaging and aimed at real-time support for surgery by remote diagnosis of frozen sections. Many hospitals and clinics have an application for the remote practice of pathology, particularly in the area of reading frozen sections in support of surgery, commonly called anatomic pathology. The goal is to project the expertise of the pathologist into the remote setting by giving the pathologist access to the microscope slides with an image quality and human interface comparable to what the pathologist would experience at a real rather than a virtual microscope. A working prototype of a virtual microscope has been defined and constructed which has the needed performance in both the image quality and human interface areas for a pathologist to work remotely. This is accomplished through the use of telerobotics and an image quality which provides the virtual microscope the same diagnostic capabilities as a real microscope. The examination of frozen sections is performed a two-dimensional world. The remote pathologist is in a virtual world with the same capabilities as a "real" microscope, but response times may be slower depending on the specific computing and telecommunication environments. The TelePath system has capabilities far beyond a normal biological microscope, such as the ability to create a low power image of the entire sample using multiple images digitally matched together; the ability to digitally retrace a viewing trajectory; and the ability to archive images using CD ROM and other mass storage devices.

Improving preventive care for women: impact of a performance improvement program in a family practice office.

PURPOSE: In 1993, the Family Medicine Center (FMC) at the Medical Center of Delaware (MCD) implemented a performance improvement (PI) program to increase rates of adult preventive care. This program included adoption of evidence-based preventive care guidelines, education of physicians and nurses regarding these guidelines, development of a flow sheet for tracking preventive care on office charts, and adoption of a policy of nurse review to alert physicians of which preventive services are due. The purpose of this study was to determine whether this PI program had a positive impact on the delivery of preventive care to female patients of the FMC. METHODS: A retrospective chart review was conducted for 280 female patients of the FMC ages 40-75. We determined the percentage of women who had received appropriate screening for breast and cervical cancer as well as appropriate immunizations, according to the guidelines of the US Preventive Services Task Force. These percentages were compared for the three year time periods before and after the PI program was implemented (July, 1990-June, 1993 and July, 1993-June, 1996). RESULTS: Prior to implementation of the PI program, PAP smears were completed in 65.3 percent of women, mammograms in 70.3 percent, breast exams in 44.2 percent, flu shots in 29.6 percent and tetanus shots in 15.5 percent. After the program, these percentages increased for all preventive services, especially PAP smears which increased by 12 percent and tetanus shots which tripled. After the program, the rate of flu shots was still low at 37 percent. CONCLUSIONS: After implementation of a PI program at the FMC, performance of preventive care for female patients improved for both cancer screening and immunizations. This improvement suggests that an office-based strategy can improve performance of preventive care, especially if it incorporates the active involvement of nursing staff. However, there is still room for further improvement, notably with completion of flu shots. These improvements will likely require additional strategies, such as computerized tracking systems and preventive care reminders to patients.

Fine mapping reveals that promotion susceptibility locus 1 (Psl1) is a compound locus with multiple genes that modify susceptibility to skin tumor development.

Although it is well known that the majority of human cancers occur as the result of exposure to environmental carcinogens, it is clear that not all individuals exposed to a specific environmental carcinogen have the same risk of developing cancer. Considerable evidence indicates that common allelic variants of low-penetrance, tumor susceptibility genes are responsible for this interindividual variation in risk. We previously reported a skin tumor promotion susceptibility locus, Psl1, which maps to the distal portion of chromosome 9, that modified skin tumor promotion susceptibility in the mouse. Furthermore, Psl1 was shown to consist of at least two subloci (i.e., Psl1.1 and Psl1.2) and that glutathione S-transferase alpha 4 (Gsta4), which maps to Psl1.2, is a skin tumor promotion susceptibility gene. Finally, variants of human GSTA4 were found to be associated with risk of nonmelanoma skin cancer. In the current study, a combination of nested and contiguous C57BL/6 congenic mouse strains, each inheriting a different portion of the Psl1 locus from DBA/2, were tested for susceptibility to skin tumor promotion with 12-O-tetradecanoylphorbol-13-acetate. These analyses indicate that Psl1 is a compound locus with at least six genes, including Gsta4, that modify skin tumor promotion susceptibility. More than 550 protein-coding genes map within the Psl1 locus. Fine mapping of the Psl1 locus, along with two-strain haplotype analysis, gene expression analysis, and the identification of genes with amino acid variants, has produced a list of fewer than 25 candidate skin tumor promotion susceptibility genes.