This group includes neurological and neuromuscular disorders, such as fragile X syndrome (FXS; MIM# 300624), which is caused by the expansion of CGG trinucleotide repeats in the 5′ untranslated region of the fragile X mental retardation 1 gene (FMR1; MIM# 309550; Eichler et al., 1994; Yrigollen et al., 2012; Nolin et al., 2014). Here, FMR1 is linked to fragile X syndrome.