Apolipoprotein A-I is the major protein present in High Density Lipoprotein (HDL). A high level of HDL has been shown to correlate with lower risk of atherosclerosis and heart disease. Apo A-I has multiple roles including transporting cholesterol from plasma membranes into HDL, activating the enzyme that converts cholesterol to cholesterol esters, and allowing the liver to recognize HDL.

Here it is shown in cartoons view with the chains color-coded. Chain A is red, chain B is green, chain c is blue, and chain d is yellow.

PDB ID 1AV1 (PDB)