GRN and frontotemporal dementia: Epidemiological studies have all established that the C9orf72 mutation is the major genetic cause of familial ALS (more frequent than superoxidose dismutase 1 mutations), and comparable in frequency to GRN mutations in familial FTD cases (DeJesus-Hernandez et al., 2011; Renton et al., 2011; Chiò et al., 2012; Hsiung et al., 2012; Majounie et al., 2012; Cruts et al., 2013; van der Zee et al., 2013).