PR-619 is a non-selective, reversible inhibitor of deubiquitinases/deubiquitylases (DUBs) and ubiquitin-like isopeptidases. PR-619 inhibits many deubiquitinating enzymes in the 5-20 μM range. Additionally, PR-619 is cell permeable and treatment of cells results in the accumulation of both ubiquitin and sumo-conjugated proteins.