Discovery of intronic hexanucleotide repeat expansions of the C9ORF72 gene in a significant proportion of patients with amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD)(1,2) was an important step for research into these disorders. The C9ORF72 genetic variant is more common than other described mutations and, unlike patients with mutations in SOD1, C9ORF72-ALS clinically and pathologically resembles the more numerous sporadic form.(3) However, progress has been limited by lack of understanding of the function of the C9ORF72 locus in health and disease. It is unknown whether the expansion causes disease by a gain of toxicity, or whether it disrupts expression of the wild-type protein encoded by the C9ORF72 gene, or some combination of both mechanisms.(1,2,4.)