CD33 and Alzheimer disease: Alzheimer’s disease (AD) is the most common neurodegenerative disease and multiple genes important for maintaining normal microglia functions, such as triggering receptor expressed on myeloid cells 2 (TREM2), apolipoprotein E (APOE), complement receptor 1 (CR1), cluster of differentiation 33 (CD33), and ATP-binding cassette transporter A7 (ABCA7) have been genetically linked to AD6.