Ubiquitylation-that is, the covalent attachment of ubiquitin polypeptides to target proteins-involves the sequential action of the E1 ubiquitin-activating enzyme, E2 ubiquitin-conjugating enzymes, and E3 ubiquitin-protein ligases. Similarly, the conjugation of ubiquitin-like proteins is thought to occur through parallel, but nonidentical, cascades. This concept of strict separation of these modification pathways is now being challenged by the evidence, showing that there are enzymes that play a role both in ubiquitylation and isgylation.