APOE (Apolipoprotein E): APOE is a protein mainly produced by the liver and astrocytes in the brain that plays a crucial role in lipid metabolism and transport, particularly within the central nervous system (CNS). It assists in clearing lipoproteins from the bloodstream and redistributes cholesterol and other lipids to cells through interaction with cell surface receptors. APOE occurs in three common isoforms—APOE2, APOE3, and APOE4—each influencing lipid metabolism differently, with APOE4 having specific implications for disease risk.