Fragile X Syndrome

The fragile X syndrome (McKusick 309550;
other designations: fraX syndrome, X-chromosomal
mental retardation with fragile site on
the X chromosome, Martin–Bell syndrome) is
the most frequent form of hereditarymental retardation
in males, with a frequency of about
1:2000–4000 individuals. The responsible
mutation usually consists of an increased number
of unstable trinucleotide repeats. Unlike in
classic X-chromosomal inheritance, there are
males without manifestations, and a large proportion
of female carriers showpartial manifestations.