GSK343 is a potent and selective small molecule inhibitor of histone methyltransferase EZH2 with an IC50 of 4 nM. It showed 60 fold selectivity against EZH1, and >1000 fold selectivity against other histone methyltransferases. GSK343 inhibits H3K27 methylation in HCC1806 breast cancer cells with an IC50 of <200 nM. GSK343 significantly suppressed the growth of EOC cells cultured in 3D matrigel extracellular matrix (ECM), which more closely mimics the tumor microenvironment in vivo. Notably, it induces apoptosis of EOC cells in 3D but not 2D culture. In addition, GSK343 significantly inhibited the invasion of EOC cells.