Apolipoprotein E serves as a ligand for low density receptors and participates in the transport and redistribution of cholesterol and other lipids. Other functions include immunoregulation and cell growth modulation and differentiation. Apo E is thought to be involved in tissue repair as increased amounts of the protein are found at sites of peripheral nerve injury and regeneration. A mutant form is associated with familial type III hyperlipoproteinemia. The concentration of Apo E in normal plasma is 5 mg per 100 ml. In addition to facilitating solubilization 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