Ubiquitin-like molecules (UBLs), such as SUMO1 (UBL1), are structurally related to ubiquitin and can be ligated to target proteins in a similar manner as ubiquitin.1,2 However, covalent attachment of UBLs does not result in degradation of the modified proteins.. Like ubiquitin, UBLs are synthesized as precursor proteins, with 1 or more aa following the C-terminal glycine-glycine residues of the mature UBL protein. Thus, the tail sequences of the UBL precursors need to be removed by UBL-specific proteases such as SUMO1-specific protease 1 (SUSP1) prior to their conjugation to target proteins.