Inhibition by CMP6 of STAT5 activity in cells expressing JAK1 and/or JAK3 mutants. HEK293 cells were transiently cotransfected with JAK1 (WT, Y652H mutant, or both in equimolar concentration to mimic heterozygous vs homozygous configuration), JAK3 (WT, V674A mutant, or both), IL-9Rα, and γc, in addition to the STAT5-responsive luciferase reporter construct pLHRE and the pRLTK plasmid as transfection control. Cells were maintained in the presence of increasing concentrations of pan-JAK inhibitor CMP6 or left untreated. Twenty-four hours posttransfection, cells were lysed and subjected to dual luciferase activity assay. Raw values were standardized with BCR-ABL–induced STAT5 activity. Histograms are means ± standard error of the mean of 5 independent experiments performed in triplicate. A 1-way analysis of variance test was used to determine P values between the heterozygous and the 2 homozygous configurations for each CMP6 concentration (*P < .05, ** P < .01, ***P < .001).