A potent, highly selective, S-adenosyl-methionine(SAM)-competitive, small-molecule inhibitor of EZH2 methyltransferase activity. GSK126 is highly selective against other methyltransferases and multiple other protein classes, even 150-fold more selective for the closest EZH1 (Kiapp = 89 nM) and more than 1,000-fold selective for 20 other human methyltransferases, including both SET-domain-containing and non-SET domain-containing methyltransferases. It decreases global H3K27me3 levels and reactivates silenced polycomb repressive complex 2 (PRC2) target genes.