PCSK9 inhibition by monoclonal antibody. Illustration of a molecule of the enzyme proprotein convertase subtilisin/kexin type 9 (PCSK9, white and pink) bound to a monoclonal antibody (blue and pale blue). PCSK9 binds to the receptor for low-density lipoprotein particles (LDL). The LDL receptor (LDLR), on liver and other cell membranes, binds and initiates ingestion of LDL-particles from extracellular fluid into cells. Blocking PCSK9 can lower blood LDL-particle concentrations. When there are high levels of LDL ('bad' cholesterol) in the blood it builds up on the sides of blood vessels hardening them, a condition named atherosclerosis. Monoclonal antibodies that bind to and inhibit PCSK9 are being used to reduce the amount of cholesterol in the blood by improving the liver's ability to recycle LDLRs.