The JAK1^R724H and the JAK1^A634D mutants, but not JAK1^R879C, confer IFN-β hypersensitivity in BW5147 and BaF3 cells. (A) BW5147 (4000) or (B) BaF3 (3000) cells stably transduced by JAK1 wild-type (WT) or different JAK1 mutants were seeded in 96-well plates and stimulated by 1% IFN-β or mock/control HEK293 supernatants. After 72 hours, tritiated thymidine was added to the cells for 4 hours, and thymidine incorporation was measured. Results are mean ± SD of biologic triplicates. Similar results were obtained in 3 independent experiments in BW5147 and BaF3 cell lines.