PD0325901 is the selective inhibitor of mitogen activated protein kinase kinase (MEK or MAPKK) with IC50 about 1nM against activated MEK1 and MEK2. Its potency, solubility and effectiveness are all much better than CI-1040. Anticancer activity of PD0325901 has been demonstrated for a broad spectrum of human tumor xenografts. When using with GSK-3β inhibitor CHIR99021, PD0325901 could prevent cell differentiation and sustain ES cell self-renewal.