The MAP kinase kinase inhibitor, U0126, distinguishes F36VMpl- and F36VFGFR1-induced cell proliferation. (A) F36VMpl-transduced (•) and F36VFGFR1-transduced (▴) mouse marrow cells were cultured in suspension in the presence of AP20187 (100 nM) and a range of concentrations of the following inhibitors: U71322 (phospholipase Cγ), SU6656 (Src kinases), Ly294002 (phosphatidylinositol-3 kinase), AG490 (Jak2), Jak inhibitor-1 (all Jak family members), and U0126 (MAP kinase kinase). Each inhibitor was evaluated at least twice with very similar results. (B) F36VFGFR1 was unable to sustain the ex vivo expansion of marrow cells from Stat5a/b^ΔNΔN mice (▵), whereas sustained growth was observed using marrow cells from C57Bl6/J mice (▴). No expansion of marrow cells from wild-type or Stat5a/b^ΔNΔN mice occurred in the absence of CID (data not shown). Initial growth of Stat5a/b knockout mouse marrow cells in response to F36VFGFR1 signaling was confirmed in a second independent experiment. (C) Intracellular flow cytometry shows only faint Erk1/2 phosphorylation following CID stimulation (solid lines) of both F36VFGFR1-transduced (left) and F36VMpl-transduced (right) marrow cells. Dashed line indicates before stimulation.