JAK1^A634D potentiates the antitumoral effect of type I IFN in vivo. Rag2^−/− mice received an intramuscular injection of an IFN-α6T construct or of an empty vector. Two days later, 10⁶ BW5147 leukemia cells expressing or not JAK1^A634D were injected intraperitoneally (10 mice/group). Kaplan-Meier curves of tumor development are shown. A Gehan-Breslow-Wilcoxon test shows that JAK1^A634D expression significantly delayed tumor formation compared with control cells (P < .034). These results were reproduced in 3 experiments with either Rag2^−/− or severe combined immunodeficient mice.