The SUMO tag is the covalent attachment of the protein, small ubiquitin-related modifier (SUMO), to a fusion protein of interest for its expression and purification. It is reported that SUMOylation enhances the expression and solubility of connected proteins and decreases proteolytic degradation. The tag can also be removed post-translationally by SUMO protease 1 and by native SUMO proteases in a eukaryotic system. For immunological detection of SUMO-tagged proteins, antibodies to the SUMO epitope are also available.