IRF-4/8 DKO progenitors are more sensitive to GM-CSF–stimulated proliferation than single KO cells. (A) Lineage-depleted BM cells (3 × 10⁵ cells/well) were cultured for 4 days in the presence of GM-CSF, and viable cells were counted to determine the proliferative response of lin⁻ progenitors to GM-CSF. *P < .035, comparisons between IRF-8 KO and wild-type cells at all 3 concentrations of GM-CSF. **P < .008, comparison between IRF-4/8 DKO and IRF-8 KO cells. (B) Growth of lin⁻ progenitors in the presence of 0.1 ng/mL GM-CSF. BrdU incorporation was monitored. *P < .001, comparison between wild-type and IRF-8 KO cells at day 2. **P < .013 for comparison between wild-type and IRF-4/8 DKO cells at both day 2 and day 3.