UBE2I (Ubc9) is a member of the E2 family and is homologous to ubiquitin-conjugating enzymes, but is specific for the conjugation of SUMO to a variety of target proteins. This E2 is unusual in that it interacts directly with protein substrates that are modified by SUMOylation, and may play a role in substrate recognition. UbcH9 can mediate the conjugation of all SUMOs to a variety of proteins including RanGAP1, IkBa, and PML without the requirement of an E3 ligase. This enzyme has a mutation of the active site from cysteine to serine which abolishes the ability of UbcH9 to transfer ubiquitin to an accepting E3 protein. Ideal for use as a negative or competitive control, or to study protein-protein interactions.