Ubiquitin-conjugating enzyme E2 E3 (UBE2E3 alias UbcH9) is a protein that in humans is encoded by the UBE2E3 gene. The deduced 207-amino acid protein shares over 94% amino acid identity with the UBC domains of class III E2s, UbcH6, UBE2E2, UbcM2, UbcM3, and UbcD2. But the N-terminal extension exhibited little homology among these, except for UbcM2, which showed 100% identity, and which is thought to be a mouse counterpart. The UBE2E3 form a thioester bond with ubiquitin (Ub) in an E1-dependent manner and mediates the transfer of ubiquitin from a ubiquitin-activating enzyme (E1) to a substrate protein or E3 ligase.