The view rapidly changed when, besides the already discovered genes causing exclusively ALS—superoxide dismutase-1 (SOD1)—or FTD—progranulin (GRN) and microtubule-associated protein Tau (MAPT)—the GGGGCC hexarepeat expansion in chromosome 9 open reading frame 72 (C9orf72) gene was identified in 30%–50% of familial ALS, around 25% of familial FTD and about 5% of sporadic ALS and FTD, indicating that C9orf72 is the major genetic factor in both conditions. The gene discussed is MAPT; the disease is frontotemporal dementia.