The process of SUMO-1 conjugation is similar to that seen with ubiquitin and other forms of post-translational protein modification. Like ubiquitin, SUMO-1 is conjugated to its target protein by the coordinated action of ubiquitin conjugation enzymes E1, E2 and E3. Ubc9 (or ube2M) is a highly conserved, 158 amino acid protein that acts as a SUMO-1 conjugating enzyme. Ubc9 binds to target proteins through their SUMO-1-CS (consensus sequence) domains and interacts with SUMO via the structurally conserved amino-terminal domain. Localization of Ubc9 to the nucleus and the nuclear envelope allows this enzyme to catalyze target protein sumoylation and regulate target protein nucleocytoplasmic transport and transcriptional activity. Ubc9 target proteins include a host of proteins (RAD51, RAD52, p53 and c-Jun) that regulate the cell cycle, DNA repair, and p53-dependent processes.