[Clinical, cytogenetic and molecular aspects of fragile X syndrome].

With a prevalence of 1:1400, the fragile-X syndrome is the most frequent hereditary cause of mental retardation in boys. In young children the clinical picture differs from that in adults. Some deterioration of intelligence level and disturbances of social behaviour are especially common in prepubertal patients. Cytogenetic results are sometimes difficult to interpret. Evaluating the results of chromosomal investigations on 211 mentally retarded males, we found that about 5% of the retarded males showed a X-chromosomal abnormality which resembled fra Xq27, but appeared not to be associated with the fragile-X syndrome. On the other hand, also false negative results were reported if 50 cells were analysed. Since almost half of the carriers do not express fra Xq27, recurrent risks are difficult to establish. The development of DNA-markers for carrier detection and prenatal diagnosis is an important development towards better diagnostic possibilities and a better understanding of the fragile site. The results of recombinant-DNA investigations in 67 members of a large family with 18 retarded males and 7 retarded females are discussed.