TREM2 and Alzheimer disease: For AD, in addition to well-known genes like APOE and TREM2, GWAS has uncovered additional genetic variants associated with immune regulation, many of which are specifically expressed in microglia, such as MS4As, CD33, SPI1, and INPP5D, or are enriched in microglial cells, like CR1, ABCA7, and CLU.