A class of protein components which can be found in several lipoproteins including HIGH-DENSITY lipoproteins; VERY-LOW-DENSITY lipoproteins; and chylomicrons. Synthesized in most organs, Apo E is important in the global transport of lipids and cholesterol throughout the body. Apo E is also a ligand for LDL receptors (receptors, ldl) that mediates the binding, internalization, and catabolism of lipoprotein particles in cells. There are several allelic isoforms (such as E2, E3, and E4). Deficiency or defects in Apo E are causes of hyperlipoproteinemia type iii.