However, the G4C2 (GGGGCC) hexanucleotide repeat expansions in the C9ORF72 gene is the most common genetic cause of ALS and FTD (11, 12), and although the expansion mechanism is uncertain, it is suggested that the cause of disease in FTD includes “gain-of-toxicity” or reduction in function of the C9ORF72 protein (13). The gene discussed is C9orf72; the disease is frontotemporal dementia.