Prolonged expression of c-fos mRNA in primitive hematopoietic stem cells from Mx–c-fos mice. Lin⁻Sca-1⁺ BM cells were cultured with SCF, IL-3, and IL-6 in the presence (200 U/mL) or absence of IFN-/β for 7 hours (A), or by the addition of IFN-/β (200 U/mL) on day 0 and day 4 (B). Levels of c-fos mRNA were measured by RT-PCR analysis followed by Southern blotting, as described in Materials and Methods. G3PDH mRNA served as an internal control for the amount of RNA.