ApoE belongs to a group of proteins that bind reversibly with lipoprotein and play an important role in lipid metabolism. In addition to facilitating solublization of lipids, these proteins help to maintain the structural integrity of lipoproteins, serve as ligands for lipoprotein receptors, and regulate the activity of enzymes involved in lipid metabolism. Significant quantities of ApoE are produced in liver and brain and to some extent in almost every organ. ApoE exists in three major isoforms; E2, E3, and E4, which differ from one another by a single amino-acid substitution. Individuals heterozygous for the ApoE4 allele are at higher risk of late-onset Alzheimer's disease. Recombinant human ApoE4 is a 34.4 kDa protein containing 300 amino acid residues.