LDL and receptor degradation with PCSK9. Computer illustration showing the degradation of a low-density lipoprotein molecule (LDL, upper right), LDL receptor (LDLR, centre-left) and a molecule of the enzyme proprotein convertase subtilisin/kexin type 9 (PCSK9, 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 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. However, if PCSK9 does not bind, the receptor can return to the surface of the cell 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.