Computer illustration showing the action of a proprotein convertase subtilisin/kexin type 9 (PCSK9) inhibitor antibody (green) on the degradation of a low-density lipoprotein molecule (LDL, round) and LDL receptor (LDLR, orange) when bound to a PCSK9 enzyme molecule (blue). When there are high levels of LDL cholesterol in the blood it builds up on the sides of blood vessels hardening them, a condition named atherosclerosis. This narrows the blood vessels and may block them. LDL receptors in cell membranes recognise and bind (upper left) to LDL molecules to remove them from the bloodstream. When PCSK9 binds to an LDLR, the receptor and LDL particle are absorbed by the cell and destroyed (lower left). However, if PCSK9 does not bind, only the LDL particle is destroyed and the receptor can return to the surface of the cell (lower right) and remove more cholesterol. 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.