The CGG repeats within the FMR1 gene predispose it to instability and the occurrence of size mosaicism, in which individuals present with different CGG repeat allele sizes such that some cells carry a full mutation allele while others carry a premutation allele; a situation that is common among individuals with FXS (Loesch et al., 2004; Lokanga et al., 2013). The gene discussed is FMR1; the disease is fragile X syndrome.