The full mutation in the FMR–1 gene of male fragile X patients is absent in their sperm

Fragile X syndrome is characterized at the molecular level by amplification of a (CGG) n repeat and hypermethylation of a CpG island preceeding the open reading frame of the fragile X gene ( FMR–1 ) located in Xq27.3. Anticipation in this syndrome is associated with progressive amplification of the (CGG) n repeat from a premutation to a full mutation through consecutive generations. Remarkably, expansion of the premutation to the full mutation is strictly maternal. To clarify this parental influence we studied FMR–1 in sperm of four male fragile X patients. This showed that only the premutation was present in their sperm, although they had a full mutation in peripheral lymphocytes. This might suggest that expansion of the premutation to the full mutation in FMR–1 does not occur in meiosis but in a postzygotic stage.