Fragile sites in human chromosomes II: demonstration of the fragile site Xq27 in carriers of X-linked mental retardation.

Demonstrating the Xq27 fragile site in men with X-linked mental retardation and in obligate carriers is a continuing problem. I report two additional families with this disorder (Families F and G) and present cytogenetic data on females from four families (D-G). These data and those previously published suggest that there are two types of families in regard to fragile Xq expression in carrier females. One type shows no apparent phenotypic effect in the female and the demonstration on the fragile Xq becomes more difficult with increasing age, whereas the second type is associated with some phenotypic effect (ie, reduction in mental ability), and the fragile Xq can be demonstrated regardless of age.