Apolipoproteins are proteins that bind and transport lipids in blood and tissues. The complex of protein and lipid is a lipoprotein. Several major apolipoproteins have been described which differ in their structure, physicochemical behavior, function and distribution. Apolipoprotein E (ApoE) binds to the LDL receptor and serves as a ligand for receptor-mediated endocytosis.
Genetic defects of apolipoproteins cause various abnormalities in lipid metabolism and increased susceptibility to heart disease. Three common alleles of ApoE (e2/e3/e4) have been identified and are expressed codominantly to generate 6 possible genotypes. ApoE allelic frequencies vary among different populations.