Folate-dependent fragile site at Xq28

What it means

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

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).

Rare diseases that can present with this2

The range is how often this sign appears among people diagnosed with each disease — it is not a probability of having any of them. Many of these signs also occur in common, non-rare conditions, and Orphanet separately records 1 disease where this sign is specifically absent. A sign on this page is a conversation to have with a clinician, not a conclusion.

Part of the broader category