In this genetic association study, variants with high functional effect were observed in participants with Alzheimer disease but not in controls in NOTCH3, a gene previously associated with cerebral autosomal-dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL), and TREM2 (Q33X) that in homozygous form causes Nasu-Hakola disease. Here, NOTCH3 is linked to early-onset autosomal dominant Alzheimer disease.