FMR1 and fragile X syndrome: ASD is predominantly a polygenic disorder, although some cases have been linked to mutations in the FMR1 gene, which is expanded in FXS (McCary and Roberts, 2013), and others to repeat expansions in DMPK, FXN, and HTT, which cause DM1, FRDA, and HD, respectively (Piras et al., 2020; Trost et al., 2020).