A cell-permeable triazine compound that directly targets human E2 Ubiquitin-conjugating enzyme Rad6B/HHR6B catalytic site via noncovalent interactions and effectively inhibits Rad6B-Ub thioester bond formation as well as subsequent substrate ubiquitination without affecting UbcH5B/UBE2D2-catalyzed BCA2 ubiquitination. Shown to inhibit the proliferation of high Rad6B-expressing MDA-MB-231 (IC50 = 6uM in 72h), but not the low Rad6B-expressing, non-transformed MCF10A, via G2/M arrest and apoptosis induction. Two known cellular Rad6 substrates, beta-catenin and PCNA, are reported to be downregulated following 24h inhibitor treatment in MDA-MB-231 cultures (0.5 to 5uM).