Constitutively elevated phosphorylation of Erk1/2 in RUNX1-ETO cells. (A) Representative immunoblot showing lineage-negative BM cells transduced with MIP or RE and drug selected for 48 hours. Cells were serum and growth factor starved for 6 hours. GM-CSF was added at 50 ng/mL. (B) Quantification of band intensities by densitometry of the blots shown in panel A. Intensities of bands from phosphorylated Erk1/2 were normalized to Erk1/2. (C) Representative immunoblot of βc^+/+ and βc^−/− cells transduced with RE. Cells were starved and stimulated as in panel A.