With the exception of rare deletions and intragenic FMR1 mutations [12,13], in most FXS cases, the loss of FMRP is caused by a CGG triplet repeat expansion in the 5′ untranslated region of exon 1 of FMR1 to more than 200 repeats, which is considered a full mutation (FM). The gene discussed is FMR1; the disease is fragile X syndrome.