C9orf72 and frontotemporal dementia: Mutations in the FUS gene are now shown to account for an additional5% of familial ALS cases and some cases of FTD.30 Recently, the most convincing direct molecular link betweenALS and FTD has been the identification of a large, intronic hexanucleotideexpansion in the previously uncharacterized C9orf72 gene of unknownfunction in families with ALS, FTD, and overlapping syndrome.31-35 This mutation accounts for approximately 40% of familialALS, 10% of sporadic ALS, 5% of sporadic FTD, and up to 80% of familial ALS-FTDcases, thus making it the most common cause of ALS and FTD.