SUMO Activating Enzyme Subunit 1 (SAE1) is the highly conserved human ortholog of yeast ubiquitin-activating enzyme E1-like (UBA2). These SUMO-activating (E1) enzymes are critical for the enzymatic attachment of SUMO molecules to a target protein by a post-translational modification process termed SUMOylation. The ATP-dependent E1 enzyme charges the SUMO by forming a high-energy thiol ester intermediate which is transferred to the UBE2I/Ubc9 SUMO-conjugating (E2) enzyme. The second step is the trans-esterification reaction whereby SUMO is transferred to Cys93 of UbcH9. UBE2I/Ubc9 is the only known E2 that is able to mediate the conjugation of SUMO to lysine residues on a variety of cellular targets, usually in the absence of a ubiquitin ligase (E3). Although UBE2I/Ubc9 can directly recognize and modify lysine residues contained in a SUMOylation motif, E3-like factors most likely facilitate the SUMOylation of specific substrates. Conjugation of the ubiquitin-like modifier SUMO requires the activities of the heterodimeric E1 (Aos1/Uba2) and the UbcH9 E2 enzyme. The dimeric activating enzyme utilizes ATP to adenylate the C-terminal glycine residue of SUMO-1 (also SUMO-2 and SUMO-3), forming a high-energy thiolester bond with the cysteine residue of Uba2 and the release of AMP and PPi. The second step is the trans-esterification reaction whereby SUMO-1 is transferred to Cys93 of UbcH9.