A potent histone H3-lysine 27 (H3K27) methyltransferase EZH2 inhibitor (IC₅₀ = 4 nM). Diplays 1000-fold selectivity for other HMTs except EZH1 (60-fold selectivity). GSK343 inhibits H3K27 methylation in HCC1806 cells with an IC₅₀ of <200nM (measured by immunofluorescence).