Ab-MuLV–transformed Jak1(+/–) cells react with cell death or growth arrest to IFN treatment. (A) Growth curves for Jak1(–/–) cells in the absence and presence of IFN-α or IFN-γ (i). Growth curves for Jak1(+/–) cells in the absence and presence of IFN-α or IFN-γ. Cells react either with growth inhibition (ii) or cell death (iii) to IFN treatment. ⋄ indicates control; ▪, IFN-α; and ▴, IFN-γ. (B) Tumor-derived cells from Jak1(+/–) and Jak1(–/–) tumors were explanted in culture in the absence and presence of IFN-α or IFN-γ; 24 hours thereafter, the cell cycle distribution was analyzed. In 9 of 20 samples investigated, cells explanted from Jak1(+/–) tumors had at least partially lost the ability to react to IFN-γ, but only one tumor sample lost the ability to react with growth arrest to IFN-α treatment. None of the Jak1(–/–)–deficient cells reacted to IFN treatment. □ indicates control; ▦, IFN-α; and ▪, IFN-γ. Four tumor samples of each group are shown.