Y559 activates ERK via SFK signaling. (A) Csf1r^−/− ivGMPs expressing indicated mutants were generated as described in Figure 4. Cells were stimulated with CSF-1 for 5 minutes and then stained against p-ERK. (B) Cells were pre-incubated with either a MEK (10 µM U0126) or SFK (2 µM SU6656) inhibitor for 1 hour prior to CSF-1 stimulation. Representative FACS plots of transduced and untransduced cells within the same sample are shown. Numbers represent average fold values ± SD (n = 3). p-ERK, phosphorylated ERK.