The mutation—similar to FXS—is due to an expansion of a CCG repeat beyond 200 in the 5′UTR of the AF4/FMR2 family member 2 (AFF2, also called FMR2), which leads to hypermethylation of the CpG island upstream of FMR2 and transcriptional gene silencing (Knight et al, 1993; Gecz et al, 1996; Gu et al, 1996). This evidence concerns the gene AFF2 and fragile X syndrome.