C9orf72 and Friedreich ataxia: Prominent examples include Friedreich ataxia (FRDA) caused by a GAA expansion in intron 1 of the FXN gene6, amyotrophic lateral sclerosis (ALS) caused by a GGGGCC expansion in intron 1 of the C9ORF72 gene7, and myotonic dystrophy type 2 (DM2) caused by a CCTG expansion in intron 1 of the ZFN9 gene8.