A potent histone H3-lysine 27 (H3K27) methyltransferase EZH2 inhibitor (IC₅₀ = 2 nM). UNC1999 is >1000-fold selective for other HMTs except EZH1 (22-fold selectivity). It inhibits H3K27 methylation in MCF10A cells with an IC₅₀ of 124 nM as measured by immunofluorescence.