C9orf72 and amyotrophic lateral sclerosis: The strongest evidence for considering ALS and FTLD as one disease, however, comes from the discovery that C9orf72 mutations are a prevalent cause of ALS, ALS-FTLD, and FTLD (Kwiatkowski et al., 2009; Vance et al., 2009) and are most likely responsible for approximately 40% of ALS and for approximately 25% of familial FTLD (Van Blitterswijk et al., 2012).