Proprotein convertase subtilisin/kexin type 9 (PCSK9) is an enzyme belonging to the proteinase K subfamily of the secretory subtilase family. It is synthesized as a soluble zymogen that undergoes autocatalytic intramolecular processing in the endoplasmic reticulum. PCSK9 binds to the receptor for low-density lipoprotein (LDL) cholesterol, inducing LDLR degradation. Variants of PCSK9 can reduce or increase circulating cholesterol. Other variants are associated with a rare autosomal dominant familial hypercholesterolemia (HCHOLA3). The mutations increase its protease activity, reducing LDLR levels and preventing the uptake of cholesterol into the cells.