When this expansion is greater than 200 repeats (known as the full mutation), the FMR1 promoter becomes hypermethylated, which prevents the expression of FMR1. Deletions and sequence variants within FMR1 result in a very small fraction of FXS cases [4-6], arguing that the loss of FMR1 function is the cause of FXS. This evidence concerns the gene FMR1 and fragile X syndrome.