Experimental design of transplants. (A) Three classes of embryos result from the genetic crosses. X¹ is an outcross of Nf1^{+/− -}to β_c^−/− mice. This is to establish Nf1 and β_c null alleles within the same animal. X² is another outcross to β_c^−/− mice to homozygose the β_c allele in Nf1^+/−, β_c^−/− mice. By intercrossing, Nf1^+/−, β_c^−/− mice X³, the 2 classes of embryos are generated. (B) To test the effect loss of β_c has on Nf1^−/−-induced MPD, Nf1^+/−, β_c^−/− mice are intercrossed and fetal livers from embryos of the correct genotype are harvested. Lethally irradiated recipients are injected with 2 million fetal liver cells and allowed a 4- to 8-week recovery period. These primary recipients are then killed to provide bone marrow for secondary recipient transplantation.