Small Ubiquitin-like Modifier or SUMO proteins are a family of small proteins that are covalently attached to and detached from other proteins in cells to modify their function. SUMOylation is a post-translational modification involved in various cellular processes, such as nuclear-cytosolic transport, transcriptional regulation, apoptosis, protein stability, response to stress, and progression through the cell cycle. There are 3 confirmed SUMO isoforms in humans: SUMO-1, SUMO-2 and SUMO-3. SUMO-2 and -3 show a high degree of similarity to each other and are distinct from SUMO-1.