Recipients of Nf1^−/−β_c^−/− bone marrow have increased CFCs and CFU-Ss. (A) Spleen and bone marrow cells from 5 secondary mice given transplants with Nf1^−/−, β_c^−/− or 4 secondary mice given transplants with Nf1^+/−, β_c^−/− fetal stem cells were assayed for CFU-GMs, -Gs, -Ms, and -GEMMs and erythroid burst-forming units. Spleen cells from animals given transplants do not show a significant difference from sample to sample, P = .164. Bone marrow plated from 5 mice given transplants with Nf1^−/−, β_c^−/−or Nf1^+/−, β_c^−/− fetal stem cells have a significantly higher number of cells capable of colony formation than the Nf1^+/−, β_c^−/− transplants, P < .001. (B) Spleen and bone marrow cells from 6 mice given transplants with Nf1^−/−, β_c^−/−or 9 mice given transplants with Nf1^+/−, β_c^−/− fetal stem cells were injected intravenously into mice irradiated with 750 rads to assay for CFU-Ss. Injected spleen cells do not show a significant difference from sample to sample, P = .228. Bone marrow cells in animals given Nf1^−/−, β_c^−/− transplants have a significantly higher number of CFU-S progenitor cells than the Nf1^+/−, β_c^−/− transplants, P < .001. Error bars indicate SE.