The apolipoproteins are a structurally-unrelated group of proteins that have some association with the transport of lipids in blood. Apolipoproteins, together with phospholipids, cholesterol and triglycerides, form spherical particles with a lipid/hydrophobic center and a apolipoprotein coat. The apolipoprotein coat promotes aqueous solubility and serves as a ligand for lipoprotein receptors. HDL may contain apolipoproteins A, C, D, E, J, L and M, while LDL contains apolipoproteins B and E.