A fragile site is non-staining gaps and breaks present on the mammalian chromosomes. Fragile X syndrome is the most common form of inherited mental retardation in hu-mans because of unequivocally clinical significance. The expression frequency of folate sensitive fragile site is evaluated in prometaphase \ metaphase chromosomes obtained from peripheral blood lymphocytes from clinically diagnosed mentally retarded patients. The present study shows expression frequency of such breaks in 5.8 % of abnor-mal cell population designated as "rare" fragile site i.e. Xq27, after karyotypic correlation with standard ideogram and to the clinical features, which is slightly higher as reported earlier observations by various laboratories, suggesting that folic acid an essential component required for DNA synthesis lacking either in diet or unknown factors present in the environment.