Figure 1: Schematic structure of HDL HDL appears in the form of spherical particles, composed of a hydrophobic center [essentially cholesteryl ester (CE) with a small quantity of triglycerides (TG)]. The HDL is covered by a monolayer of lipids (phospholipids and unesterified cholesterol) and different apolipoproteins (Apos). The 4 main Apos in human HDLs, in order of decreasing abundance, are ApoA-I, ApoAII, ApoA-IV, and ApoE. The association of HDL with many minor proteins and lipids influence its important functions.