Familial X-linked mental retardation and fragile X chromosomes in two Swedish families.

X-linked mental retardation (MR) associated with a fragile X chromosome was found in two Swedish families. The fragile X chromosome was demonstrated in 5/5 boys with mental retardation. Clinical data on four of these boys are presented. In one of the families, the mental retardation was associated with macro-orchidism, large hands and large, folded ears. In the other family, macro-orchidism was not seen, possible because the boys were younger. Fragile site X chromosomes were also seen in three obligate carriers. A summary of earlier published cases of X-linked MR associated with the fragile X chromosome is given.