PCSK9 (proprotein convertase subtilisin/kexin type 9) is a serine protease belonging to the PCSK family. It consists of a catalytic domain and an extracellular proprotein convertase subtilisin/kexin type 9 domain, primarily expressed and secreted in the liver. PCSK9 regulates the degradation of low-density lipoprotein receptors (LDLR), affecting cholesterol metabolism. It promotes the internalization of LDLR from the cell surface by binding to LDLR and directing it towards lysosomal degradation, thereby reducing the number of LDLR on the cell surface and increasing LDL cholesterol levels in the blood. Gene-specific expression and mutations, such as gain-of-function or loss-of-function variants, directly impact PCSK9 activity and are associated with familial hypercholesterolemia and cardiovascular disease risk. PCSK9 inhibitors have become effective therapeutic agents for lowering LDL levels in the blood.