Section of human apolipoprotein A-I, molecular model. This is a fragment of a high-density lipoprotein (HDL) molecule known as an apolipoprotein. HDLs are proteins that bind to lipid (fat) molecules and transport them around the body. They are sometimes called 'good cholesterol' because their role in fat metabolism contributes to the body's cardiovascular health. Apolipoproteins transport dietary lipids from the intestine to the liver, and synthesised lipids to other tissues for storage or metabolism. Apolipoprotein A-I is a common variety of these proteins. The atoms in this model are represented by coloured spheres; carbon is green, hydrogen is white, oxygen is red, nitrogen is blue, and sulphur is yellow.