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clathrin 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.