TZ9 is a cell-permeable triazine compound that inhibits the human E2 ubiquitin-conjugating enzyme Rad6B. TZ9 selectively docks to the Rad6B catalytic site, inhibits Rad6B-induced histone H2A ubiquitination, downregulates intracellular β-catenin, induces G2/M arrest and apoptosis, and inhibits proliferation and migration of metastatic human breast cancer cells (IC50 = 6 μM).