Lipoprotein particles contain a hydrophobic core of lipids within a shell of polar lipids and proteins. Lipoproteins are vehicles for the transport of hydrophobic substances through the plasma, such as cholesterol. Their surfaces contain signals regulating the transport of the lipids they carry into and out of specific cells. The receptors on target cells for LDL, for example, the major carrier of cholesterol in the blood, are localized in clathrin coated pits, so that binding of LDL leads to receptor mediated endocytosis. The vesicle containing LDL will subsequently fuse with a lysosome.