Fragile sites can be defined as heritable specific loci on human chromosomes that exhibit non-random gaps, constrictions or breaks when chromosomes are exposed to specific cell culture conditions. Over 120 different fragile sites have been identified in the human genome. Some of these sites are sensitive to folate, that is, they can be induced by a culture medium deficient in folic acid and thymidine, and hence a medium with lowered levels of dTTP or dCTP, two immediate components of DNA, or by a medium enriched either in methotrexate, an inhibitor of folate metabolism, or in fluorodeoxyuridine, an inhibitor of thymidylate synthesis. The folate-dependent fragile phenotype occurs if more than a critical number of CCG/CGG repeats are present (e.g., > 230 repeats for FRAXA).

The presence of a folate sensitive fragile site at chromosome Xq28.

HP:0003564