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, especially within the central nervous system (CNS). It facilitates the removal of lipoproteins from the bloodstream and redistributes cholesterol and other lipids to cells through interaction with cell surface receptors. APOE has three common isoforms—APOE2, APOE3, and APOE4—each influencing lipid metabolism differently, with APOE4 notably associated with increased disease risk.