Cargo molecules have to be included in carrier vesicles of different forms and sizes to be
transported between organelles. During this process, a limited set of proteins, including the
coat proteins COPI, COPII and clathrin, carries out a programmed set of sequential
interactions that lead to the budding of vesicles. A general model to explain the formation of
coated vesicles is starting to emerge but the picture is more complex than we had imagined.