APOE (Apolipoprotein E): APOE is a protein primarily produced by the liver and astrocytes in the brain that plays a key role in lipid metabolism and transport, especially in the central nervous system (CNS). It helps clear lipoproteins from circulation and redistributes cholesterol and other lipids to cells through interactions with cell surface receptors. APOE exists in three common isoforms—APOE2, APOE3, and APOE4—each affecting lipid metabolism differently, with APOE4 having particular implications for disease risk.