Apolipoprotein E, a main apoprotein of the chylomicron, binds to a specific receptor on liver cells and peripheral cells and is essential for the normal catabolism of triglyceride-rich lipoprotein constituents. ApoE exists in three major isoforms; E2, E3, and E4, which differ from one another by a single amino-acid substitution. Compared with E3 and E4, E2 exhibits the lowest receptor binding affinity. Defects in ApoE are a cause of hyperlipoproteinemia type III due to increased plasma cholesterol and triglycerides levels which are the consequence of impaired clearance of chylomicron and VLDL remnants.