CD33 and Alzheimer disease: Furthermore, in support of the role of early neuroinflammation in AD, variants of highly expressed microglial genes (e.g., triggering receptor expressed on myeloid cells 2 (TREM2), CD33, membrane spanning 4-domains A6A (MS4A6), and ATP binding cassette subfamily A member 7 (ABCA7)), which mediate important functions of microglia, have been identified as risk factors for the development of AD.