JAK1^A634D confers hypersensitivity to IFN-β in BW5147 and BaF3 cells. (A) BW5147 (4000) or (B) BaF3 (3000) cells stably transduced by JAK1^WT or JAK1^A634D, with intact or mutated Y107A FERM domain (JAK1^A634D/Y107A), 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. In parallel 10⁶ BW5147 or BaF3 cells were starved for 4 hours, lysed, and subjected to Western blot analysis. Total JAK1 and β-actin levels were assessed with anti-JAK1 and anti–β-actin antibodies. (C) BW5147 cells (4000) stably transduced by JAK1^WT or JAK1^A634D, with intact or mutated Y107A FERM domain (JAK1^A634D/Y107A), were seeded in 96-well plates and stimulated by IFN-β HEK293 supernatants at different dilutions, starting from 1%. 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. WT indicates wild-type.