Alzheimer’s disease (AD), dementia with Lewy bodies (DLB), and frontotemporal lobar degeneration (FTLD) develop in the elderly population and share the accumulation of pathological proteins (tau and amyloid-β in AD, α-synuclein in DLB, and TDP-43, tau, and FUS in FTLD). The gene discussed is MAPT; the disease is frontotemporal dementia.