ATF3-null MCs have diminished proliferation and enhanced apoptosis after growth factor withdrawal. WT or ATF3-null MCs were cultured in IL-3/SCF for 5 weeks. Mature MCs were starved of growth factors for 6 hours, washed, and reseeded in the indicated concentrations of IL-3 (in ng/mL) or SCF (in ng/mL). (A) Proliferation of MCs. Cells were incubated with growth factors for 48 hours then stained with 4,5-dimethylthiazol-2-yl-3,5-diphenylformazan, thiazolyl blue formazan to evaluate proliferation. (B) MC apoptosis. Cells were stained with annexin V/PI, and apoptosis was determined by flow cytometry. Results are expressed as mean ± SEM for 3 independent experiments. *P < .05 by comparison to WT.