The most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) in populations of European ancestry is a hexanucleotide repeat expansion in the chromosome 9 open reading frame 72 (C9orf72) gene.1,2 In the United States, C9orf72 mutations account for ∼10% of ALS and 10%–15% of FTD diagnoses, including ∼30%–40% of familial ALS3 and ∼50% of familial FTD,4 as well as many individuals with both FTD and ALS. This evidence concerns the gene C9orf72 and amyotrophic lateral sclerosis.