Classic familial inheritance is observed in 5%–10% of ALS cases and in 30%–50% of FTD cases; whereas some genes like SOD1 or MAPT are predominantly linked to ALS or FTD, respectively, disease-causing variants in a number of genes have been identified in ALS/FTD families (see Table 1). The gene discussed is MAPT; the disease is frontotemporal dementia.