Small Ubiquitin-like Modifier 3 (SUMO3), also known as SMT3A, is synthesized as a 103 amino acid (aa), propeptide with a predicted 11.5 kDa. SUMO3 contains a two aa C-terminal prosegment. Human SUMO3 shares 83% sequence identity with mouse SUMO3. Di-SUMO3 represents two wild-type recombinant human SUMO3 molecules linked via Lys11, which is the point of attachment for the C-terminal glycine residue of the preceding SUMO3. Di-SUMO3 can be used as a substrate for SUMO-specific isopeptidases (SENPs) and DeSUMOylating Isopeptidase 1 that cleave the isopeptide linkage between two adjacent SUMO3 molecules. Di-SUMO3 can also be used to investigate mechanisms of binding and recognition by SUMO-activating (E1) enzymes, SUMO-conjugating (E2) enzymes, SUMO E3 ligases and other proteins that contain SUMO binding domains. SUMOs are a family of small, related proteins that can be enzymatically attached to a target protein by a post-translational modification process termed SUMOylation. Unlike SUMO1 which is usually conjugated to proteins as a monomer, SUMO2 and SUMO3 form high molecular weight polymers on proteins. All SUMO proteins share a conserved ubiquitin domain and a C-terminal diglycine cleavage/attachment site. Following prosegment cleavage, the C-terminal glycine residue of SUMO3 is enzymatically attached to a lysine residue on a target protein.