ApoE (Apolipoprotein E) is a protein primarily involved in the metabolism and regulation of lipoprotein particles. It consists of multiple functional domains and is primarily located on low-density lipoprotein (LDL) and high-density lipoprotein (HDL) particles, widely distributed in the liver, brain, and other tissues. ApoE plays a crucial role in regulating cholesterol transport, immune modulation, and cell repair processes. Humans have three ApoE alleles: ε2, ε3, and ε4, which determine structural and functional differences in the ApoE protein. The ApoE ε4 allele is associated with an increased risk of Alzheimer's disease, cardiovascular diseases, and lipid metabolism disorders. Mutations or dysregulation in ApoE can affect the metabolism and function of lipoproteins, contributing to the development of various diseases.