IFN-γ deficiency reflects the phenotype of Jak1 deficiency. (A) Colony formation of Ab-MuLV–transformed fetal liver cells from IFN-γ(+/–) and IFN-γ(–/–) embryos. No differences were observed. One representative experiment of each genotype is shown. (B) Tumor formation in SCID mice. A total of 1 × 10⁶ transformed cells were injected. The top panel shows the 8 tumors that developed upon injection of IFN-γ(+/–) cells. The bottom panel depicts 8 of the 13 tumors that developed out of IFN-γ(–/–) cells.