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

X-linked mental retardation associated with expression of a fragile site at Xq27.3 has attracted much interest because transmission can occur through phenotypically normal males. Several theories have been proposed to explain the segregation pattern. Linkage analysis in affected families indicates a high frequency of recombination around this site in some families, although in others the genetic relationships are quite different and closer linkage between bridging markers is suggested. The problems associated with the clinical and cytogenetic analyses of this fascinating disorder await the results of detailed molecular approaches.