A protein that is the major constituent of the ‘coat’ of the coated pits and coated vesicles formed during endocytosis of materials at the surface of cells (see endosome). The clathrin molecules are arranged in a localized polyhedral lattice on the membrane, which subsequently invaginates to form the coated pit and vesicle. When the coated vesicle has delivered its contents within the cell, the clathrin-coated membrane is recycled and returns to the cell surface. A similar process also tranfers materials between membranous organelles within the cell.