A selective, SAM-competitive small molecule inhibitor of EZH2 methyltransferase activity (Ki = 0.57 nM; IC50 = 9.9 nM versus that of EZH1: Ki = 89 nM; IC50 = 680 nM); inhibits global H3K27me3 levels, inhibiting the proliferation of EZH2 mutant DLBCL cell lines (IC50 = 28-61 nM) as well as the growth of EZH2 mutant DLBCL xenografts in mice receiving a daily dose of 50 mg/kg.