SUMO is transferred from cysteine-173 of UBA2 to cysteine-93 of UBC9 (UBE2I) in a transthiolation reaction (reviewed in Wang and Dasso 2009, Wilkinson and Henley 2010, Hannoun et al. 2010, Gareau and Lima 2010). UBC9 is the only known E2 enzyme for SUMO and on certain substrates such as RanGAP1 may act without the requirement of an E3 ligase.