The fragile X syndrome.

Males who possess the fragile X chromosome, a marker on the end of the X chromosome at position Xq27.3, have a distinct form of mental retardation, which has come to be known as the fragile X [fra(X)] syndrome. This X-linked syndrome is the most common Mendelian form of mental retardation. Males who inherit the fra(X) chromosome are usually moderately to severely retarded. There are also normal carrier men who are nonpenetrant for the mutation. They transmit the mutation to their daughters, who can have affected sons. The development of methods for detecting fra(X) in blood and amniotic fluid cells has allowed for population screening and prenatal diagnosis of the syndrome. New methods using DNA probes and restriction fragment length polymorphisms (RFLPs) are being applied to study the inheritance of fra(X). They have revealed apparent genetic linkage heterogeneity. Molecular studies to analyze the underlying mutation are underway to define the structural basis.