LDL receptor. Computer illustration of a low-density lipoprotein (LDL), or 'bad' cholesterol, receptor (LDLR) protein. 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 recognise and bind to LDL molecules to remove them from the bloodstream.