They account for 60–70 heritable neuropathologies Chen et al. (2025); Mirkin (2007); Paulson (2018), including Huntington’s disease (CAG repeats on the short arm of chromosome 4p16 in the Huntingtin (HTT) gene) Walker (2007); MacDonald et al. (1993), Fragile X Syndrome (CGG repeats within the 5′ UTR in the FMR1 gene) Saldarriaga et al. (2014); Kremer et al. (1991), Kennedy’s disease (CAG repeats on the Xq11-q12 band of the long arm of the X chromosome) Fischbeck (1997), myotonic dystrophy and several spinocerebellar ataxias Ellegren (2004). The gene discussed is FMR1; the disease is fragile X syndrome.