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