JAK1 pseudokinase mutations demonstrate sensitivity to JAK inhibitors. (A) Representative graphs of the dose-response curves (72-hour sensitivity) for different JAK inhibitors on Ba/F3 cells expressing BCR-ABL, JAK1 A634D, and JAK1 R629_S632delinsSA (upper panels). Graphs showing mean IC₅₀ (lower left panel) and area under the dose-response curve (AUC; lower right panel) for different JAK inhibitors on Ba/F3 cells expressing BCR-ABL, JAK1 A634D, and JAK1 R629_S632delinsSA (3 independent experiments). (B) Immunoblot analysis of IL-3–independent Ba/F3 cells expressing BCR-ABL, JAK1 A634D, and JAK1 R629_S632delinsSA treated with vehicle or 50 nM or 100 nM of ruxolitinib for 4 hours (n = 2 replicates). (C) Sensitivity of peripheral blood and bone marrow specimens from the patient with JAK1 R629_S632delinsSA to JAK inhibitors, represented by IC₅₀, in comparison with control samples obtained from healthy donors (HD) and specimens from a larger cohort of patients with MPNs) (the horizontal lines denote median IC₅₀ of HD and MPN specimens). ****P < .0001, ***P < .001, 1-way analysis of variance. p-, phosphorylated.