UbiQ-057 (Ub-PA) is an activity-based probe for deubiquitinating enzymes (DUBs) and contains propargyl amide (PA) as an electrophile. UbiQ-057 can be used for activity profiling experiments and determining DUB inhibitor specificity. It has two unique capabilities: 1) It forms a covalent linkage with (the active site Cys residue of) a DUB that can be cleaved by acid treatment (5% aq. TFA), allowing for proteomic analyses. 2) It targets all three major DUB families: UCH, USP and OTU.