Ser585 or Tyr577 of β_c are not required for GM-CSF–mediated cell cycle progression. CTL-EN cells expressing the wt GM-CSF receptor or the indicated mutants were washed and plated out at 1 × 10⁵ cells/mL in RPMI containing 10% FCS and either no factor (nil), 20 ng/mL positive control cytokine IL-2 (IL-2), or 50 ng/mL GM-CSF (GM-CSF) for 20 hours, after which the cells were pulsed for 4 hours with 10 μM BrdU. The cells were then fixed, stained with anti–BrdU-Fluos, and counterstained with propidium iodide (PI). Cells positive for both BrdU and PI were analyzed by flow cytometry, and duplicate samples ± SD are plotted.