Although methylation underpins classical FXS and accounts for a majority of FXS cases, FXS is ultimately a result of the loss of function of FMRP, as demonstrated by patients diagnosed with FXS possessing a nonsense, missense, or frameshift mutation in FMR1 yet normal CGG repeat length (Wells, 2009; Suhl and Warren, 2015; Tekendo-Ngongang et al., 2021). Here, FMR1 is linked to fragile X syndrome.