Fragile site Xq27 and mental retardation. Clinical and cytogenetic manifestation in heterozygotes and hemizygotes of five kindreds.

Clinical and cytogenetic data of five kindreds with X-linked mental retardation and a methotrexate-inducible fragile site at the distal long arm of the X chromosome fra(X)(q27) are reported; comprising a total of 26 individuals studied cytogenetically, 10 hemizygotes, five obligate heterozygotes, seven facultative heterozygotes, and four normal males, i.e., fathers and brothers of affected hemizygotes. The heterozygotes in two of these sibships show partial phenotypic and/or mental manifestation. Two of them, who are obligate heterozygotes, expressed fra(X)(q27) in 23% and 16% of their metaphases at the ages of 27 and 53 years. In the obligate and facultative heterozygotes, who are mentally normal, the marker X chromosome could not be detected in lymphocyte cultures. We conclude from these findings that the occurrence of fra(X)(q27) might correlate with the phenotypic expression in heterozygotes rather than with the age of the individual.