The preprotein convertase chymotrypsin (PCSK9) is a member of the proteinase K subfamily of the secreted Bacillus subtilis protease family. It undergoes intramolecular autocatalytic processing in the endoplasmic reticulum and is synthesized as a soluble zymogen.PCSK9 binds to the low-density lipoprotein (LDL) cholesterol receptor and induces degradation of the LDLR.PCSK9 variants can lower or raise circulating cholesterol. Other variants are associated with the very rare autosomal dominant familial hypercholesterolemia (HCHOLA3). Mutations enhance their protease activity, lowering LDLR levels and preventing cellular uptake of cholesterol.