CaSR expression in normal parathyroid and PHPT: new insights into pathogenesis from an autopsy-based study.

PURPOSE: Calcium sensing receptor (CaSR), on the surface of normal parathyroid cells, is essential for maintaining serum calcium levels. The normal pattern of CaSR immunostaining remains undefined and is presumptively circumferential. Given the physiological variation in serum calcium, we postulated that CaSR expression could not be uniformly circumferential. Also, cytoplasmic expression has not been evaluated either in normal or pathological tissues. We studied normal parathyroid tissues derived from forensic autopsies and those rimming parathyroid adenomas for membranous and cytoplasmic CaSR immunoexpression. Results were compared with primary hyperparathyroidism (PHPT) to look for any pathogenetic implications. MATERIALS AND METHODS: We evaluated 34 normal parathyroid tissues from 11 autopsies, 30 normal rims, 45 parathyroid adenoma, 10 hyperplasia, and 7 carcinoma cases. Membranous expression was categorized complete/incomplete and weak/moderate/strong; scored using Her2/Neu and Histo-scores; predominant pattern noted. Cytoplasmic expression was categorized negative/weak/moderate/strong; predominant intensity noted. RESULTS: Normal autopsy-derived parathyroid tissues were Her2/Neu 3 + , but incomplete membranous staining predominated in 85%. Their immune-scores were significantly more than the cases (p < < 0.05). The mean histo-score of normal rims was intermediate between the two (p < < 0.05). Cytoplasmic expression was strong in all autopsy-derived tissues, weak/negative in hyperplasia (100%), moderate in 16% adenomas, and 43% carcinomas. CONCLUSIONS: Normal autopsy-derived parathyroid tissues showed strong but predominantly incomplete membranous expression. Surface CaSR expression decreased in PHPT and is probably an early event in parathyroid adenoma, seen even in normal rims. Whether there is a defect in CaSR trafficking from the cytoplasm to the cell surface in adenoma and carcinoma needs further evaluation.

Serum otolin-1 as a biomarker for benign paroxysmal positional vertigo: a case-control study.

OBJECTIVES: This study aimed to evaluate serum otolin-1 levels in patients with benign paroxysmal positional vertigo and to compare these levels with healthy individuals. METHOD: This was a case-control study. After obtaining institutional ethical committee clearance, the serum level of otolin-1 was calculated in adult individuals (18-75 years old) who were divided into group 1 (patients presenting with benign paroxysmal positional vertigo) and group 2 (healthy patients without benign paroxysmal positional vertigo as the control group). Data analysis was carried out to compare the serum levels in the cases and controls. A p-value less than 0.05 was considered significant. RESULTS: A total of 70 age-matched individuals (cases, n = 40; controls, n = 30) were included in the study. The mean serum level of otolin-1 was 636.8 pg/ml (range, 259-981 pg/ml) in the group of patients with benign paroxysmal positional vertigo and 236.2 pg/ml (range, 189-370 pg/ml) in the control group. The difference was statistically significant (p = 0.0000). CONCLUSION: The serum levels of otolin-1 in patients with benign paroxysmal positional vertigo are significantly higher compared with individuals without benign paroxysmal positional vertigo.

Adiponectin correlates with obesity: A study of 159 childhood acute leukemia survivors from India.

BACKGROUND: Acute lymphoblastic leukemia survivors are predisposed to obesity. However, the exact underlying mechanisms are not known. AIMS: The study was done to assess the role of biomarkers of obesity in acute leukemia survivors. SETTINGS AND DESIGNS: This is a cross-sectional study conducted at All India Institute of Medical Sciences in survivors of acute leukemia who had completed treatment at least 1 year before enrollment in this study. MATERIALS AND METHODS: Obesity was studied by determining the body mass index. Potential biomarkers were studied by assessing serum leptin, resistin, and adiponectin by enzyme-linked immunosorbant assay, and the results were compared in obese versus nonobese survivors. STATISTICAL ANALYSIS: Descriptive analysis for baseline demographic factors and Student's t-test for comparing the mean levels of biomarkers among the obese and nonobese survivors. RESULTS: One hundred and fifty-nine acute leukemia patients were enrolled in this study with a median follow-up of 36.8 months. The median age was 10 (range: 3-18) years, and 123 (77.3%) patients were males. The overall prevalence of overweight/obesity was 26.4%, and this was similar in acute myeloid leukemia and acute lymphoblastic leukemia sub-groups (26.2% vs. 27.3%, P = 0.9). Mean serum leptin and resistin were similar in obese and nonobese leukemia survivors (3.7 vs. 2.85 pg/mL, P = 0.064; 8.01 vs. 9.33 ng/mL, P = 0.36). However, mean serum adiponectin was significantly lower in obese leukemia survivors (7.97 vs. 11.5 µg/mL, P = 0.023). CONCLUSIONS: Obese leukemic survivors had lower serum adiponectin levels than nonobese survivors. However, serum resistin and leptin levels were similar in the two groups.

Growth from birth to adulthood and peak bone mass and density data from the New Delhi Birth Cohort.

UNLABELLED: Growth in early life may predict adult bone health. Our data showed that greater height and body mass index (BMI) gain in utero and infancy are associated with higher peak bone mass, and greater BMI gain in childhood/adolescence with higher peak bone density. These associations are mediated by attained adult height and BMI. INTRODUCTION: To study the relationship of height and BMI during childhood with adult bone mineral content (BMC), areal density (aBMD) and apparent density (BMAD, estimated volumetric density). METHODS: Participants comprised 565 men and women aged 33-39 years from the New Delhi Birth Cohort, India, whose weight and height were recorded at birth and annually during infancy (0-2 years), childhood (2-11 years) and adolescence (11 years-adult). Lumbar spine, femoral neck and forearm BMC and aBMD were measured using dual X-ray absorptiometry; lumbar spine and femoral neck BMAD were calculated. RESULTS: Birth length, and height and height gain during infancy, childhood and adolescence were positively correlated with adult BMC (p≤0.01 all sites except birth length with femoral neck). Correlations increased with height from birth to 6 years, then remained constant for later height measurements. There were no associations with BMAD. BMI at birth, and during childhood and adolescence was also positively correlated with BMC (p < 0.01 all sites). BMI at 11 years, and BMI gain in childhood and adolescence, were correlated with aBMD and BMAD (p < 0.001 for all); these correlations strengthened with increasing age of BMI measurement. The associations with height and BMI in early life became non-significant after adjustment for adult height and BMI. CONCLUSIONS: Greater skeletal growth and BMI gain in utero and during infancy are associated with higher peak BMC, and greater BMI gain in childhood and adolescence is associated with higher peak aBMD and BMAD. These associations are mediated by the attainment of adult height and BMI, respectively.

Assessment of the scale, coverage and outcomes of the Avahan HIV prevention program for female sex workers in Tamil Nadu, India: is there evidence of an effect?

BACKGROUND: Avahan, the India AIDS Initiative, a large-scale HIV prevention program, using peer-mediated approaches and STI services, was implemented for high-risk groups for HIV in six states in India. This paper describes the assessment of the program among female sex workers (FSWs) in the southern state of Tamil Nadu. METHODS: An analytical framework based on the Avahan impact evaluation design was used. Routine program monitoring data, two rounds of cross-sectional biological and behavioural surveys among FSWs in 2006 (Round 1) and 2009 (Round 2) and quality assessments of clinical services for sexually transmitted infections (STIs) were used to assess trends in coverage, condom use and prevalence of STIs, HIV and their association with program exposure. Logistic regression analysis was used to examine trends in intermediate outcomes and their associations with intervention exposure. RESULTS: The Avahan program in Tamil Nadu was scaled up and achieved monthly reported coverage of 79% within four years of implementation. The cross-sectional survey data showed an increasing proportion of FSWs being reached by Avahan, 54% in Round 1 and 86% in Round 2 [AOR=4.7;p=0.001]. Quality assessments of STI clinical services showed consistent improvement in quality scores (3.0 in 2005 to 4.5 in 2008). Condom distribution by the program rose to cover all estimated commercial sex acts. Reported consistent condom use increased between Round 1 and Round 2 with occasional (72% to 93%; AOR=5.5; p=0.001) and regular clients (68% to 89%; AOR=4.3; p=0.001) while reactive syphilis serology declined significantly (9.7% to 2.2% AOR=0.2; p=0.001). HIV prevalence remained stable at 6.1% between rounds. There was a strong association between Avahan exposure and consistent condom use with commercial clients; however no association was seen with declines in STIs. CONCLUSIONS: The Avahan program in Tamil Nadu achieved high coverage of FSWs, resulting in outcomes of improved condom use, declining syphilis and stabilizing HIV prevalence. These expected outcomes following the program logic model and declining HIV prevalence among general population groups suggest potential impact of high risk group interventions on HIV epidemic in Tamil Nadu.

Validation of self-reported smoking status by measuring serum cotinine levels: an Indian perspective.

BACKGROUND: Serum cotinine levels are a reliable marker of tobacco use. Few studies have validated questionnaires assessing smoking and exposure to environmental tobacco smoke (ETS) against serum levels. We undertook such a study in industrial workers in India. METHODS: We chose 426 individuals by stratified random sampling from a database of 3397 individuals surveyed at New Delhi for the cardiovascular disease surveillance programme in a large industrial setting. Questionnaires assessing details of smoking practices and duration of exposure to ETS (if any) were administered. Cotinine levels were measured in the blood samples of these individuals. RESULTS: The study population comprised 142 nonsmokers not exposed to ETS, 142 non-smokers exposed to ETS and 142 active smokers. Cotinine levels among nonsmokers not exposed to ETS were non-detectable; and for non-smokers exposed to ETS and active smokers, the median (interquartile range) levels were non-detectable (non-detectable to 46.1 ng/ml) and 336 ng/ml (204-500 ng/ml), respectively. The best combined sensitivity (91%) and specificity (87.2%) yielded a cotinine cut-off level of 40.35 ng/ml to differentiate active smokers from non-smokers not exposed to ETS and those exposed to ETS (area under the curve 0.902). The cut-off cotinine level was estimated at 10.95 ng/ml using a similar analysis (sensitivity 43%, specificity 82%; area under the curve 0.64) to distinguish non-smokers not exposed to ETS from those exposed to ETS. The misclassification rate was estimated at 19% and 57.1% among self-reported non-smokers not exposed to ETS and those exposed to ETS, respectively. CONCLUSIONS: Obtaining a history of tobacco use is an accurate method of detecting smokers in epidemiological studies whereas serum cotinine levels accurately differentiate smokers from non-smokers. However, a brief questionnaire assessing passive exposure to smoke has poor sensitivity in distinguishing non-smokers exposed to ETS from those not exposed to ETS.

Impact of alcohol on coronary heart disease in Indian men.

BACKGROUND: Moderate alcohol consumption is known to be protective against coronary heart disease (CHD). However, the INTERHEART study, a case-control study of acute myocardial infarction (MI) patients, revealed that alcohol consumption in South Asians was not protective against CHD. We therefore planned to study cardiovascular risk factor and CHD prevalence among male alcohol users as compared to age matched lifetime abstainers. METHODS: The subjects for this study were recruited from a cross-sectional survey carried out among employees and their family members aged 20-69 years in 10 medium-to-large industries from diverse sites in India, using a stratified random sampling technique. Information on education, behavioral, clinical and biochemical risk factors of CHD and alcohol use was obtained through standardized instruments. CHD diagnosis was based on Rose Questionnaire or a prior physician diagnosed CHD. RESULTS: A total of 4465 subjects were present or past alcohol users. The mean age of alcohol users and lifetime abstainers was 42.8+/-11.0 years and 42.8+/-11.1 years, respectively (p=0.90). Systolic blood pressure and diastolic blood pressure were significantly higher in alcohol users (128.7+/-17.6 mmHg/80.1+/-11.3 mmHg) as compared to lifetime abstainers (126.9+/-15.9 mmHg/79.5+/-10.3 mmHg, p<0.01). Fasting blood sugar in alcohol users (98.7+/-30.5 mg%) was also significantly higher than lifetime abstainers (96.6+/-26.0 mg%, p<0.01). Total cholesterol was lower in alcohol users (179.1+/-41.1 mg%) as compared to lifetime abstainers (182.7+/-38.2 mg%, p<0.01). HDL cholesterol was higher in alcohol users (42.9+/-10.8 mg%) as compared to lifetime abstainers (41.3+/-10.0 mg%, p<0.01). Body mass index (BMI) was lower in alcohol users as compared to lifetime abstainers (22.7+/-4.1 kg/m2 vs. 24.0+/-3.3 kg/m2, p<0.001). Tobacco use was significantly higher in alcohol users (63.1% vs. 20.7%). The odds ratio (OR) of having CHD after adjusting for tobacco use, BMI and education was 1.4 (95%CI 1.0-1.9) in alcohol users as compared to controls. The OR was 1.2 (95%CI 0.8-1.6) in occasional alcohol users, 1.6 (95%CI 1.0-2.2) in regular alcohol users and 2.1 (95% CI 1.1-3.0) in past alcohol users as compared to controls. CONCLUSION: We did not observe an inverse (protective) association between alcohol intake and the prevalence of CHD. In contrast, our study indicated an association in the reverse direction, suggesting possible harm of alcohol for coronary risk in Indian men. This relationship needs to be further examined in large, prospective study.

Prevalence and determinants of diabetes mellitus in the Indian industrial population.

AIM: To highlight the regional difference in the prevalence of diabetes mellitus (DM) and to explore determinants in variability in the Indian industrial population. METHODS: A cross-sectional survey was carried out among the employees and their family members (10 930 individuals, mean age 39.6 years, 6764 male) of eleven medium-to-large industries from diverse sites in India, using a stratified random sampling technique. Information on behavioural, clinical and biochemical risk factors of DM was obtained, through standardized instruments. DM was diagnosed when fasting blood glucose was > or = 7.0 mmol/l and/or individuals took drug treatment for DM. Multiple logistic regression analysis was carried out to identify the potential predictors of DM. RESULT: In the 20 to 69-year-old age group, the crude prevalence of DM and impaired fasting glucose was 10.1 and 5.3%, respectively. Urban sites had a higher prevalence and awareness of DM status. Individuals in the lower education group had a high prevalence of DM (11.6%). In diabetic subjects, 38.4% were unaware that they had diabetes. Waist-circumference-to-height ratio had a higher DM predictive power than waist circumference and body mass index. The risk factors associated with overall prevalence of DM were: age, sex, low-education level, family history of DM, hypertension and overweight/obesity. Interaction of risk factors was observed only in urban high-prevalence sites. CONCLUSION: There are wide regional variations in the prevalence of DM in India. The high burden of undetected diabetes, even in settings with universal access to on-site health care, highlights the need for innovative prevention and control strategies.

Peripheral vascular disease in systemic lupus erythematosus.

With increasing longevity of lupus patients, peripheral vascular disease (PVD) has become an important cause of morbidity. With no systematic study of PVD in systemic lupus erythematosus (SLE), this study was undertaken to define the frequency and spectrum of PVD in SLE and factors affecting such an occurrence. All medium-sized peripheral arteries of bilateral upper and lower extremities were studied in 50 SLE patients using Doppler ultrasonography. PVD was defined clinically as one or more of intermittent claudication, absent/unequal pulses, gangrene or ischemic ulcers and sub-clinically as asymptomatic patients with Doppler abnormalities, with > or =50% reduction in diameter considered hemodynamically significant. Mean (SD) age of the patients was 31.6 (10.1) years. Forty-one percent were hypertensive. Dyslipidemia was found in 62%. Fifteen (30%) had Raynaud's phenomenon. Fourteen (28%) patients had PVD, of whom three had positive markers for antiphospholipid antibody (aPL) and six were asymptomatic. Ischemic ulcers were seen in eight (16%), gangrene in three (6%), femoral artery plaques in two (4%), stenosis in four (8%) and intermittent claudication in none. Dyslipidemia was found to independently affect occurrence of PVD (OR = 5.37, [95% CI 1.05-27.5], P = 0.05). The causes of PVD overlap significantly and further studies are needed to ascertain the relative contribution of each.

Risk factors related to the development of diabetes in men working in a north Indian industry.

BACKGROUND: Epidemiological and lifestyle changes have been implicated in the high burden of diabetes in urban India. However, longitudinal data on the determinants for the development of diabetes in this population are not available. We investigated the determinants for the development of diabetes in workers in an Indian industrial organization. METHODS: Two cross-sectional surveys were done, using similar methodology (Survey 1 during 1995-98 [n=2548] and Survey 2 during 2002-03 [n=2800]) among all employees (age 20-59 years) of an industrial organization. A large majority of these were men (89.5% in Survey 1 and 92.8% in Survey 2). Men with no diabetes at baseline, who participated in both the surveys (n=942), constituted the study population. Development of new-onset diabetes was defined using history and fasting glucose concentrations > or =7 mmol/L. RESULTS: The mean (SD) age of the participants at baseline was 40 (2) years. Diabetes developed in 8% of the study population over 6.8 (1.7) years. Individuals who developed diabetes had significantly higher age, blood pressure, body mass index, waist circumference, fasting and post-prandial glucose, post-prandial insulin and fasting triglyceride levels at baseline. On multivariate regression analysis, only impaired glucose tolerance (OR 3.8, 95% CI: 2.1-6.8) and waist circumference (OR 1.09, 95% CI: 1.02-1.16) predicted the development of diabetes. Presence of the metabolic syndrome, as defined by the modified National Cholesterol Education Program Adult Treatment Panel (NCEP-ATP) III and WHO criteria, increased the odds (95% CI) of developing diabetes by 2.2 (1.3-3.6) and 4.5 (2.7-7.4) times, respectively. CONCLUSION: Impaired glucose tolerance, high waist circumference and the metabolic syndrome are powerful predictors for the development of diabetes among urban Indian men.

Methods for establishing a surveillance system for cardiovascular diseases in Indian industrial populations.

OBJECTIVE: To establish a surveillance network for cardiovascular diseases (CVD) risk factors in industrial settings and estimate the risk factor burden using standardized tools. METHODS: We conducted a baseline cross-sectional survey (as part of a CVD surveillance programme) of industrial populations from 10 companies across India, situated in close proximity to medical colleges that served as study centres. The study subjects were employees (selected by age and sex stratified random sampling) and their family members. Information on behavioural, clinical and biochemical determinants was obtained through standardized methods (questionnaires, clinical measurements and biochemical analysis). Data collation and analyses were done at the national coordinating centre. FINDINGS: We report the prevalence of CVD risk factors among individuals aged 20-69 years (n = 19 973 for the questionnaire survey, n = 10 442 for biochemical investigations); mean age was 40 years. The overall prevalence of most risk factors was high, with 50.9% of men and 51.9% of women being overweight, central obesity was observed among 30.9% of men and 32.8% of women, and 40.2% of men and 14.9% of women reported current tobacco use. Self-reported prevalence of diabetes (5.3%) and hypertension (10.9%) was lower than when measured clinically and biochemically (10.1% and 27.7%, respectively). There was marked heterogeneity in the prevalence of risk factors among the study centres. CONCLUSION: There is a high burden of CVD risk factors among industrial populations across India. The surveillance system can be used as a model for replication in India as well as other developing countries.

Atherosclerosis in Asian Indians with systemic lupus erythematosus.

OBJECTIVES: Atherosclerosis has emerged as an important late complication of systemic lupus erythematosus (SLE). Asian Indians, as an ethnic group, are known to be metabolically predisposed to development of early atherosclerosis. No data on this aspect of SLE are available from Asia. This study was undertaken to find the frequency of atherosclerosis in Indian lupus patients and the factors affecting such an occurrence. METHODS: Carotid artery intimo-medial thickness (IMT) and plaque were used as markers of atherosclerosis. High-resolution B-mode ultrasonography was used to compare carotid IMT and plaque in 50 patients with SLE and 50 age- and sex-matched healthy controls. RESULTS: Patients with lupus (age 31.6+/-10.05, median 30.5 years; disease duration 52.3+/-36.7, median 46 months) exhibited a significantly greater IMT than controls (0.417+/-0.07 vs. 0.362+/-00.07 mm; p = 0.003). Carotid plaques were seen in seven (14%) cases. None of the control population had plaques (p = 0.006). On bivariate analysis, the IMT was significantly affected by age, systolic blood pressure (SBP), disease duration and menopausal status. On multivariate analysis, the only factor significantly affecting IMT was SBP. The Systemic Lupus International Collaborating Clinics/American College of Rheumatology (SLICC/ACR) score was the sole factor found to significantly affect the occurrence of plaque. CONCLUSIONS: Asian Indian lupus patients in our study, despite being relatively young and with shorter disease duration, exhibited premature atherosclerosis in the form of significantly thicker intimo-media and plaque. The factors found to affect accelerated atherosclerosis in our cohort were age, SBP, disease duration, postmenopausal status and the SLICC/ACR score.

Dynamic nature of host-pathogen interactions in Mycobacterium marinum granulomas.

Mycobacterium marinum causes long-term subclinical granulomatous infection in immunocompetent leopard frogs (Rana pipiens). These granulomas, organized collections of activated macrophages, share many morphological features with persistent human tuberculous infection. We examined organs of frogs with chronic M. marinum infection using transmission electron microscopy in conjunction with immunohistochemistry and acid phosphatase cytochemistry to better define the bacterium-host interplay during persistent infection. Bacteria were always found within macrophage phagosomes. These phagosomes were often fused to lysosomes, in sharp contrast to those formed during in vitro infection of J774 macrophage-like cells by M. marinum. The infected macrophages in frog granulomas showed various levels of activation, as evidenced by morphological changes, including epithelioid transformation, recent phagocytic events, phagolysosomal fusion, and disintegration of bacteria. Our results demonstrate that even long-term granulomas are dynamic environments with regard to the level of host cell activation and bacterial turnover and suggest a continuum between constantly replicating bacteria and phagocytic killing that maintains relatively constant bacterial numbers despite an established immune response. Infection with a mutant bacterial strain with a reduced capacity for intracellular replication shifted the balance, leading to a greatly reduced bacterial burden and inflammatory foci that differed from typical granulomas.

Granuloma-specific expression of Mycobacterium virulence proteins from the glycine-rich PE-PGRS family.

Pathogenic mycobacteria, including the agent of tuberculosis, Mycobacterium tuberculosis, must replicate in macrophages for long-term persistence within their niche during chronic infection: organized collections of macrophages and lymphocytes called granulomas. We identified several genes preferentially expressed when Mycobacterium marinum, the cause of fish and amphibian tuberculosis, resides in host granulomas and/or macrophages. Two were homologs of M. tuberculosis PE/PE-PGRS genes, a family encoding numerous repetitive glycine-rich proteins of unknown function. Mutation of two PE-PGRS genes produced M. marinum strains incapable of replication in macrophages and with decreased persistence in granulomas. Our results establish a direct role in virulence for some PE-PGRS proteins.

Thermolabile methyltetrahydrofolate reductase associated with unusual venous and arterial thromboses.

We report a patient with unusual venous and arterial thromboses in association with the common thermolabile methyltetrahydrofolate (MTHFR) variant. The patient responded directly to folate supplementation. To our knowledge, this is the first report describing hyperhomocysteinemia in association with this type of thrombosis.

Novel structure of the human GDNF gene.

Glial cell line-derived neurotrophic factor (GDNF) is the most potent known survival factor for substantia nigra neurons, which degenerate in Parkinson's disease, for spinal motoneurons, which die in Lou Gehrig's disease (ALS), and for Purkinje neurons, the critical outflow cells of the cerebellum. Moreover, targeted deletion of the GDNF gene results in renal dysgenesis and abnormal development of the enteric nervous system. GDNF mRNA is expressed in a complex temporospatial pattern in the central nervous system and the periphery, consistent with these observations. To begin elucidating mechanisms regulating the pattern of expression of GDNF, we have cloned the human gene, and characterized the promoter. The promoter is highly GC rich, and lacks canonical CCAT-box and TATA-box motifs. It contains more than 12 binding sites for known transcription factors. These cis-elements have the potential to interact with factors regulating constitutive expression (Sp1) and developmental expression (bHLH). Moreover, the promoter contains sites for binding transcription factors which respond to environmental signals, including CREB, AP2, Zif/268, NFkB, and MRE-BP. Combinatorial actions of these transcription factors may account for the extraordinarily complex expression patterns of the GDNF gene. Importantly, we demonstrate that the hGDNF gene utilizes a promoter distinct from that identified in the rodent GDNF gene, a finding with ramifications for Parkinson's disease and ALS research.

A crtB homolog essential for photochromogenicity in Mycobacterium marinum: isolation, characterization, and gene disruption via homologous recombination.

A gene essential for light-induced pigment production was isolated from the photochromogen Mycobacterium marinum by heterologous complementation of an M. marinum cosmid library in the nonchromogen Mycobacterium smegmatis. This gene is part of an operon and homologous to the Streptomyces griseus and Myxococcus xanthus crtB genes encoding phytoene synthase. Gene replacement at this locus was achieved via homologous recombination, demonstrating that its expression is essential for photochromogenicity. The ease of targeted gene disruption in this pathogenic Mycobacterium allows for the dissection of the molecular basis of mycobacterial pathogenesis.