Mutation of Tyr 577 in β_c enhances GM-CSF–responsive survival. FL cells transduced with α chain and wtβ_c or Tyr577Pheβ_c were washed and plated at 1.5 × 10⁵/mL in IMDM containing either 0.5% HI-FCS or 10% HI-FCS. Groups contained either no added factor (□), 50 ng/mL hGM-CSF (■), or a prosurvival cytokine cocktail containing IL-6 (50 ng/mL), SCF (100 ng/mL), and G-CSF (10 ng/mL) (▧). After 48 hours, cells were stained with the 4H1 anti-GMRα monoclonal antibody followed by an anti-mouse IgG-PE antibody, and with annexin V–FLUOS. Viability analysis was performed by flow cytometry as described in “Materials and methods.” The average of duplicate samples (± SD) is indicated. *P < .001; **P = .001. This experiment is representative of 3 performed.