In molecular biology, SUMO (Small Ubiquitin-like Modifier) proteins are a family of small proteins that are covalently attached to and detached from other proteins in cells to modify their function. This process is called SUMOylation (sometimes written sumoylation). 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. SUMO proteins are similar to ubiquitin and are considered members of the ubiquitin-like protein family. SUMOylation is directed by an enzymatic cascade analogous to that involved in ubiquitination. In contrast to ubiquitin, SUMO is not used to tag proteins for degradation. Mature SUMO is produced when the last four amino acids of the C-terminus have been cleaved off to allow formation of an isopeptide bond between the C-terminal glycine residue of SUMO and an acceptor lysine on the target protein. SUMO family members often have dissimilar names; the SUMO homologue in yeast, for example, is called SMT3 (suppressor of mif two 3). Several pseudogenes have been reported for SUMO genes in the human genome. Function SUMO modification of proteins has many functions. Among the most frequent and best studied are protein stability, nuclear-cytosolic transport, and transcriptional regulation. Typically, only a small fraction of a given protein is SUMOylated and this modification is rapidly reversed by the action of deSUMOylating enzymes. SUMOylation of target proteins has been shown to cause a number of different outcomes including altered localization and binding partners. The SUMO-1 modification of RanGAP1 (the first identified SUMO substrate) leads to its trafficking from cytosol to nuclear pore complex. The SUMO modification of ninein leads to its movement from the centrosome to the nucleus. In many cases, SUMO modification of transcriptional regulators correlates with inhibition of transcription. One can refer to the GeneRIFs of the SUMO proteins, e.g. human SUMO-1, to find out more. There are 4 confirmed SUMO isoforms in humans; SUMO-1, SUMO-2, SUMO-3 and SUMO-4. At the amino acid level, SUMO1 is about 50% identical to SUMO2. SUMO-2/3 show a high degree of similarity to each other and are distinct from SUMO-1. SUMO-4 shows similarity to SUMO-2/3 but differs in having a Proline instead of Glutamine at position 90. As a result, SUMO-4 isn't processed and conjugated under normal conditions, but is used for modification of proteins under stress-conditions like starvation. During mitosis, SUMO-2/3 localize to centromeres and condensed chromosomes, whereas