C9orf72 and fragile X syndrome: More than 40 refractory neurological diseases are caused by microsatellite repeat expansions, including spinal and bulbar muscular atrophy; Huntington disease (HD); Friedreich ataxia; fragile X syndrome; fragile X–associated tremor/ataxia syndrome (FXTAS); myotonic dystrophy types 1 and 2 (DM1, DM2); C9orf72-associated amyotrophic lateral sclerosis/frontotemporal dementia (C9orf72-ALS/FTD); dentatorubral-pallidoluysian atrophy; spinocerebellar ataxia (SCA) types 1, 2, 3, 6, 7, 8, 10, 12, 17, 31, 36, and 37 (Paulson, 2018); and benign adult familial myoclonic epilepsy (Ishiura et al., 2018).