The diagnosis initially considered Alzheimer’s disease (AD), frontotemporal degeneration (FTD), and corticobasal degeneration (CBD), cerebral hemiatrophy syndrome, but imaging and cerebrospinal fluid analysis excluded AD and suggested fused-in-sarcoma-associated FTD (FTLD-FUS), a subtype of the behavioural variant of FTD. The gene discussed is FUS; the disease is early-onset autosomal dominant Alzheimer disease.