Certain proteins (called apolipoproteins B and E) on the surface of lipoprotein particles are responsible for mediating the binding of cholesterol-rich particles to specific lipoprotein receptors on the surface of cells and represent a major pathway controlling blood cholesterol levels. Three important disorders of lipoprotein metabolism, which provide insights into the molecular mechanisms responsible for the elevation of specific atherogenic lipoproteins, are the following: (1) Type III hyperlipoproteinemia results from specific mutations in apolipoprotein E that prevent the normal binding of chylomicron remnants and very-low-density lipoprotein remnants to lipoprotein receptors. Patients with this disorder who have elevated levels of these remnant lipoproteins develop atherosclerosis. (2) Familial defective apolipoprotein B-100 results from a single amino acid substitution in apolipoprotein B that prevents low-density lipoprotein from binding normally to the low-density lipoprotein receptor and elevates plasma cholesterol levels. (3) Familial hypercholesterolemia, which results in elevated levels of plasma low-density lipoprotein and premature atherosclerosis, is caused by a variety of mutations in the low-density lipoprotein receptor that interfere with the normal binding of lipoproteins to this receptor. These observations not only provide insights into the mechanisms responsible for normal lipoprotein metabolism, but also highlight the potential role of specific lipoproteins in atherogenesis.