Sumoylation, the covalent attachment of a small ubiquitin-like modifier (SUMO) peptide to lysine residues of targeted substrate, has recently emerged as an important mechanism in transcriptional control. In humans and mice, the SUMO family consists of three members, SUMO-1, -2, and -3. SUMO-2 (SMT3A, Sentrin-3) and SUMO-3 (SMT3B, Sentrin-2) are similar (~95% identical), but less closely related to SUMO-1 (~50% identical). Whereas many proteins modified by SUMO-1 were identified, many as yet unidentified proteins are modified by SUMO-2/3 after exposure of cells to various stress stimuli