MS4A4A and Alzheimer disease: Risk variants of AD that are associated with microglia of the aged brain (Figure 1) are TREM2, CD33, INPP5D, HLA-DQA1, MS4A4A, CASS1, SORL1, SCIMP, SPPL2A, APOC1, and ATXN7L (Olah et al., 2018), which makes microglia a central player in recent AD research.