Loss of a c-Myb allele has modest effects on colony formation of bone marrow progenitors. Colony formation assays were performed using (A) enriched Lin⁻ marrow cells (5 × 10³ cells/plate) plated in methylcellulose in the presence of GM-CSF (P = .015); (B) Lin⁻Sca-1⁺ marrow cells (5 × 10³cells/plate) plated in methylcellulose in the presence of IL-3, IL-6, and KL (P = .008); and (C) Lin⁻Sca-1⁺Kit⁺ marrow cells (2.5 × 10⁵/plate) plated in methylcellulose in the presence of optimal (IL-3, 6 ng/mL; IL-6, 10 ng/mL; KL, 50 ng/mL) or suboptimal (1/10th) cytokine concentrations; P = .011 and P ≤ .001, respectively. Representative of 3 different experiments performed in duplicate.