Small ubiquitin-like modifiers (SUMOs) are a family of small, related proteins (Sumo-1/2/3/4) that can be enzymatically attached to a target protein by a post-translational modification process termed sumoylation, which is a major regulator of protein function in cellular processes such as nuclear transport, transcriptional regulation, apoptosis and protein stability. All SUMO proteins localize to the nucleus and are covalently conjugated, affecting protein structure, function and interactions. SUMO2 and 3 are 96% identical and are more mobile within nucleus relative to SUMO1. Specific functional differences between SUMO1 and SUMO2 and 3 remain to be identified.