Methylation mosaicism, the presence of both fully methylated and unmethylated FM alleles, as opposed to the presence of methylated FM alone has been associated with better clinical outcomes and higher IQ levels in FXS individuals [17], and the subset of these methylation mosaics and unmethylated FM males who display moderate or normal phenotypes are referred to as high-functioning fragile X males [23,24,25]. Here, PSMB5 is linked to fragile X syndrome.