Kinetics of colony formation by primitive hematopoietic stem cells. Lin⁻Sca-1⁺ BM cells (3 × 10²) from Mx–c-fos mice or control littermates were cultured with SCF, IL-3, and IL-6 (□). IFN-/β (200 U/mL) was added on day 0 (•), or on day 0 and day 4 (○). The number of colonies was scored every 4 days. Results represent mean and SD of four dishes. The data presented are representative of two independent experiments.