A potent MEK inhibitor that suppresses phosphorylation of ERK in murine colon 26 tumors with an IC50 value of 0.33 nM; suppression of ERK activation with 1 µM PD 0325901 combined with 3 µM CHIR99021 (a glycogen synthase kinase-3 inhibitor) prevents cell differentiation and sustains self renewal of mouse embryonic stem cells for at least eight passages.