Purity >98% by TLC. Cell-permeable. An irreversible ubiquitin-activating enzyme (E1) inhibitor (IC50 < 10 MM). Blocks ubiquitylation and prevents ubiquitin-mediated proteasomal degradation. Inhibits NF- Kappa B activation, blocks degradation of p53, increases p21 levels and induces apoptosis in vitro. Also causes an increase in sumoylation of proteins.