DI-591 is a potent, high-affinity and cell-permeable inhibitor of the DCN1-UBC12 interaction. DI-591 binds to DCN1 and DCN2 with Ki values of 12 nM and 10.4 nM, respectively and has no appreciable binding to DCN3, DCN4, and DCN5 proteins. DI-591 selectively inhibits neddylation of cullin 3 but has no or minimal effect on neddylation of other cullin family members.