In addition, individuals with FXS might show mosaicism at two different levels: (1) CGG repeat lengths, with some cells harboring fully expanded mutation alleles and other cells harboring more benign alleles; and (2) methylation levels, with some cells containing methylated FMR1 alleles and other cells containing unmethylated FMR1 alleles. Here, FMR1 is linked to fragile X syndrome.