Ubiquitin (UB) is a protein modifier that regulates many essential cellular processes. To initiate protein modification by UB, the E1 enzyme activates the C-terminal carboxylate of UB to launch its transfer through the E1-E2-E3 cascade onto target proteins. The E1 enzyme is the activating enzyme, to which ubiquitin is attached in an ATP-dependent reaction by a thioester bond. The E2 enzyme is the conjugating enzyme, to which the ubiquitin is transferred from the E1. The E3 is the ubiquitin ligase, which directly or indirectly catalyzes the transfer of the ubiquitin to the target protein (the substrate), with the formation of an isopeptide bond.