Computer illustration of a low-density lipoprotein (LDL), or 'bad' cholesterol, molecule (round) bound to an LDL receptor (LDLR) protein (red) in a cell membrane (bottom). The LDL molecule consists of a core of esterified cholesterol molecules surrounded by a shell of unesterified cholesterol and phospholipids. The complex structure includes carrier proteins (blue) known as apo-B proteins, which assist transport in the blood. 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.