Activation of p42/Erk2 MAP kinase in primary CD34⁺ cells measured by gel retardation assay and Western blotting (see Materials and Methods). (A) Cells were stimulated with either cytokine diluent (0.001% FCS), SCF (100ng/mL), or TPA (500 ng/mL) for the times indicated. Replicate samples were preincubated with either DMSO diluent or the PLA₂ inhibitor MAFP (5 μmol/L) for 15 minutes before cytokine stimulation. (B) Cells were incubated with cytokines as in (A) as well as GM-CSF (10 ng/mL) for the times indicated, and Erk2 activation was measured as in (A).