CFU-S12 assays were performed as described previously5, except that CFU-S colonies were counted after 12 days. In brief, to assess the frequency of CFU-S in mutant mice, total bone marrow was flushed from the femora and tibiae of mutant mice and appropriate controls. Nucleated cells were enumerated using a solution of 3% acetic acid and methylene blue and injected intravenously into 20 recipient mice that had been lethally irradiated. After 12 days the spleens were fixed in Bouin’s solution (Sigma), and the number of colonies were counted and expressed relative to the number of total bone marrow cells injected.
