In mammalian cells the clathrin coated vesicles participate in receptor mediated internalization, transport and sorting of macromolecules from the trans Golgi network to the endocytic pathway as well as from the plasma membrane to endosomes and lysosomes. The complex protein machinery controlling vesicle budding, transport and fusion are significantly conserved between the endocytic and sorting pathways. The internalization and intracellular transport are based respectively on tyrosine (YXXĆ) and dileucine (LL) signals.