X-linked mental retardation. I. Martin-Bell syndrome (report of 18 families).

Eighteen families with X-linked mental retardation (MR) with or without macroorchidism, fragile-X positive at Xq27 (Martin-Bell syndrome) have been studied clinically and cytogenetically. All 58 affected males presented variable degrees of MR, fra(X) (q27) of their peripheral lymphocytes, macroorchidism in all adult patients with the exceptions of one with microorchidism as 47,XXY sex chromosome complement and the other with borderline testes, and characteristic facial appearance. The expression of the marker X in the heterozygotes seems to be more related to the mental development rather than the age of the individual. In two families the transmission of the syndrome through unaffected males seems probable.