[Familial mental retardation and the fragile X syndrome]

In a large kindred 24 mentally retarded persons (17 males and 7 females) were chromosomally investigated. An X-chromosomal abnormality was found in all. The aberration has been described as a fragile site at the end of the long arm: fra (X)(q27). In one of the normally functioning men the abnormality was found in 6% of the cells. It is not always possible to prove the presence of the fragile site in female carriers. We failed to do so in one of the six obligate carriers. Because of the high risk for children of carriers genetic counseling is important, but may not be easy.