Purity >98%. The lysine methyltransferase EZH2 (KMT6), part of the polycomb repressive complex 2, catalyzes trimethylation of lysine 27 on histone H3 (H3K27me3) and is involved in proliferation and aggressive cell growth associated with neoplastic cells. GSK503 is an inhibitor of EZH2 that prevents the methyltransferase activity of wild-type and mutant EZH2 with similar potency (Ki app = 3-27 nM). It is more than 200-fold selective for EZH2 over EZH1 and more than 4,000-fold selective over other histone methyltransferases. GSK503 displays favorable pharmacokinetics in mice. It prevents germinal center formation and hyperplasia that is relevant to lymphoma generation and inhibits growth and metastasis of cutaneous melanomas in mice.