In this multicentre study, we measured plasma phosphorylated-tau217 in controls (n = 60), Niemann–Pick disease type C (n = 71) and Alzheimer’s disease (n = 30 positive for amyloid and negative for tau in CSF [A+T−] and n = 30 positive for both [A+T+]). The gene discussed is MAPT; the disease is early-onset autosomal dominant Alzheimer disease.