An irreversible inhibitor of ubiquitin-activating enzyme (E1; IC50 = 50 = 6.4 μM); selective for E1 over E2 but does inhibit autoubiquitination of the HECT domain E3 Nedd4 at 50 μM; inhibits proteasome-dependent and -independent ubiquitination and protein degradation at 50 μM; preferentially induces apoptosis of E1A-transformed RPE cells expressing wild-type p53 over non-transformed RPE cells at 1-20 μM; also increases net cellular sumoylation and irreversibly cross-links proteins such as JAK2 and Bcr-Abl