DNA studies of X-linked mental retardation associated with a fragile site at Xq27.3.

The fragile-X mental retardation syndrome (FRAX-MR) is one of the most prevalent X-linked diseases. In affected males and in a proportion of carrier females a fragile site at Xq27.3 [fra (X)] is detected when the lymphocytes are cultured under conditions of thymidine deprivation. The fra (X) analysis can be used in the diagnosis of only 56% of carrier females (13, 14) and prenatal diagnosis by this method is not always feasible, thus making genetic counselling of affected families difficult, and sometimes impossible. We have analysed FRAX-MR families using RFLP and four DNA probes from Xq27-Xq28. Estimation of the recombination fraction indicates that the proximal probe F9 is not significantly linked to the FRAX-MR locus while the three distal probes F8, DXS52 and DXS15 show linkage. These probes could be used in the diagnosis of FRAX-MR in those families that do not show evidence for recombination. Used in conjunction with the fra (X) analysis, the segregation studies with these probes should improve the genetic counselling of the FRAX-MR syndrome and should be useful for the genetic and molecular analysis of this unique disease.