C9orf72 and Huntington disease: They account for more than 40 neurological disorders, including HD (CAG repeat expansion in HTT), FXTAS and FXS (CGG repeat expansion in FMR1), dentatorubral‐pallidoluysian atrophy (DRPLA; OMIM# 125370; CAG in ATN1), C9orf72‐related disorders (GGGGCC in C9orf72) and some SCA.19