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Cholesterol, molecular model. Atoms are represented as rods and are colour-coded: carbon (blue), hydrogen (yellow) and oxygen (red). Cholesterol is a lipid, a fat-like substance, though chemically it is a sterol, a type of alcohol. It is an important cell chemical, and plays a crucial role in the synthesis of bile salts and hormones, and in transporting fats in the bloodstream. High blood levels of LDL (low- density lipoprotein) cholesterol compared to HDL (high-density lipoprotein) cholesterol are thought to raise the risk of atherosclerosis, the build-up of fatty deposits on artery walls. This is a major cause of heart attack and stroke. (KEYSTONE/SCIENCE PHOTO LIBRARY/DR MARK J. WINTER)