The effect of MEK inhibition on TPO-induced proliferation of BAF3/Mpl cells. BAF3/Mpl cells were cultured in the presence of various concentrations of recombinant murine TPO. The cultures also contained PD 98059 (dissolved in DMSO) at final concentrations of 50 μmol/L (▪) or 100 μmol/L (X), whereas the control cultures contained a similar volume of DMSO (○). The number of living cells was determined after 36-hour cultures using the MTT method described in Materials and Methods. The TPO-induced proliferation is presented as the percentage of maximal IL-3–induced proliferation. The abscissa is displayed in a logarithmic scale. The results represent the mean (±SD) of triplicate determination in a single representative experiment. This experiment has been performed 3 times with similar results.