One of a group of compounds consisting of a lipid combined with a protein. Lipoproteins are the main structural materials of the membranes of cells and cell organelles. They also occur in blood and lymph, being the form in which lipids are transported in these media. Cholesterol is transported in the bloodstream mainly in the form of low-density lipoproteins (LDLs) and is removed by means of LDL receptors in cell membranes; the LDLs are bound to the receptors, which are then taken into the cells. LDLs comprise about 50% cholesterol, 25% protein, and 25% lipids. Lack of LDL receptors, occurring as a genetic defect in some individuals, is a cause of high levels of cholesterol in the blood, predisposing to atherosclerosis. Plasma levels of LDL cholesterol can be reduced by dieting and exercise and/or by drugs (such as statins). Very low-density lipoproteins (VLDLs) are formed in the liver and consist largely of triglycerides (94%), with just 2% protein and 3% cholesterol. They are linked to a high risk of cardiovascular disease. High-density lipoproteins (HDLs) transport cholesterol from tissues to the liver and contain about 50% protein, 35% lipids, and 15% cholesterol. These, the so-called ‘good’ lipoproteins, effectively ‘scavenge’ cholesterol, and their concentration in blood is raised by exercise. A high ratio of LDLs to HDLs in the blood is associated with an increased risk of atherosclerosis and heart disease. See also chylomicron.