One genome-wide association study comprising 1 126 563 participants found CD33 to be one of 38 genomic risk loci.6 In contrast, a study of 788 989 participants pinpointed 75 risk loci but notably excluded CD33 from its list of those identified.8 Such disparities underscore the complexity and heterogeneity inherent in Alzheimer’s disease and reaffirm the ambiguous role of CD33 in its pathogenesis. The gene discussed is CD33; the disease is early-onset autosomal dominant Alzheimer disease.