Lipoproteins are large complexes of molecules that transport lipids (primarily triglycerides and cholesterols) through the blood. Apolipoproteins are proteins on the surface of the lipoprotein complex that bind to specific enzymes or transport proteins on the cell membranes. This directs the lipoprotein to the proper site of metabolism . Apolipoprotein CII (apoCII) is in chylomicrons (large lipoprotein particles absorbed from the gastrointestinal (GI) tract) and VLDL (large, mostly triglyceride-containing, lipoproteins that are broken down to eventually form LDL ); apoCII activates the enzyme (lipoprotein lipase) that breaks down chylomicron and VLDL molecules.