C9orf72 and frontotemporal dementia: Expansion mutations in the C9orf72 gene are one of the most common causes of familial amyotrophic lateral sclerosis (ALS) and familial frontotemporal dementia (FTD) in US and European populations (Renton et al., 2011, DeJesus-Hernandez et al., 2011, Majounie et al., 2012).