The size threshold at which a repeat becomes unstable and/or pathogenic varies widely, from the expansion of only a few trinucleotide repeats in e.g. ARX-associated infantile epileptic encephalopathy (MIM 308350) to over a thousand repeats in e.g. DMPK-associated myotonic dystrophy (MIM 160900), FXN-associated Friedreich ataxia (MIM 229300) and FMR1-associated fragile X syndrome (MIM 300624) [16], [17], [18]. The gene discussed is FMR1; the disease is fragile X syndrome.