A lumbar puncture in patient BII at 26 years of age showed markedly diminished levels of Aβ42 at 34.95 pg/mL (normal range, >190 pg/mL) and elevated phosphorylated tau at 152.3 pg/mL (normal range, <78 pg/mL) consistent with Alzheimer’s disease. The gene discussed is MAPT; the disease is early-onset autosomal dominant Alzheimer disease.