Proprotein convertase subtilisin-like kexin type 9 (PCSK9) is a newly discovered serine protease that destroys low density lipoprotein (LDL) receptors in liver and thereby controls the level of LDL in plasma. Mutations that increase PCSK9 activity cause hypercholesterolemia and coronary heart disease (CHD); mutations that inactivate PCSK9 have the opposite effect, lowering LDL levels and reducing CHD. Although the mechanism of PCSK9 action is not yet clear, the protease provides a new therapeutic target to lower plasma levels of LDL and prevent CHD.