Fragile X-linked mental retardation and the difficulties of reverse genetics.

Fragile X-linked mental retardation is an enigmatic inheritable syndrome in which severe mental retardation, a cytogenetically detectable fragile site at Xq27.3 (FraX) and a number of dysmorphic features are associated. Genetic analysis shows that the mode of inheritance is more complex than a straightforward X-linked recessive trait and probably involves a two-step process for which several models have been proposed. Early attempts at 'cloning the fragile site' provided several DNA segments lying in its general vicinity, and large scale DNA mapping methods were extensively applied in an effort to generate maps including this region. These efforts were complemented by more focussed methods such as microdissection; together these approaches have now provided a number of DNA segments within a 5 cM interval around FraX, and with the help of these new probes the site is indeed being cloned. Unravelling the nature of the sequence(s) responsible for the mental retardation syndrome will probably take some time, however.