UbcH9 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 the SUMO-1 to a variety of proteins including RanGAP1, IκBα, and PML without the requirement of an E3 ligase.