C9orf72 and amyotrophic lateral sclerosis: Cognitive or behavioral impairment is diagnosed in up to half of patients with ALS, and in 15% of patients, the clinical criteria of frontotemporal dementia (FTD) are met.1,2 In approximately 10%–15% of patients, a pathogenetic variant of one of the ALS-related genes is identified, most commonly C9orf72, SOD1, TARDBP, and FUS.