The initial translation products of SUMO1, SUMO2, and SUMO3 are precursors that have extra amino acid residues at the C-terminus (reviewed in Wang and Dasso 2009, Wilkinson and Henley 2010, Hannoun et al. 2010, Gareau and Lima 2010, Hay 2007). SUMO1 has 4 extra residues, SUMO2 has 2 extra residues, and SUMO3 has 11 extra residues. Proteolytic cleavage by SUMO peptidases (SENPs) removes the propeptide and leaves diglycine residues at the C-terminus. Each SENP has distinct preferences for certain SUMOs. SENP1 has highest activity on SUMO1; SENP2 and SENP5 have highest activity on SUMO2 (Shen et al. 2006, Reverter and Lima 2006, Mikolajczyk et al. 2007). SENP1 and SENP2 are predominantly nucleoplasmic (Bailey and O'Hare 2004, Kim et al. 2005, Zhang et al. 2002, Hang and Dasso 2002, Itahana et al. 2006) and SENP5 is predominantly nucleolar (Di Bacco et al. 2006, Gong and Yeh 2006), therefore the processing reactions are believed to occur in the nucleus. The processed SUMO is then activated by formation of a thioester bond with a cysteine residue of an E1 enzyme, UBA2 (SAE2) in a complex with SAE1. SUMO is then transferred from the E1 enzyme to an E2 enzyme, UBC9 (UBE2I).