This is consistent with the observation that the asymptomatic accumulation of β-amyloid pathology occurs for many years before the presentation of symptoms, and our result suggests that one aspect of microglial function (m114) may therefore contribute to β-amyloid accumulation, consistent with functional studies of the CD33 AD risk allele10 which affects the same trait; this effect on amyloid, in turn, contributes to cognitive decline. This evidence concerns the gene CD33 and Alzheimer disease.