The STAT5A/VVV and STAT5A/WKR mutants are defective in transcriptional activation. 293T cells were transfected with pcDNA3 vector alone, wild-type STAT5A (WT), or the indicated STAT5A mutant in the absence (−) or presence (+) of JAK2 and either wild-type (▪) or point mutant (▧) β-casein luciferase reporter. The transcriptional activation of the mutant or wild-type β-casein reporter was assessed by luciferase assay and expressed in relative light units (RLU). To control for variations in transfection efficiency, all samples were cotransfected with an hGH expression plasmid to allow normalization of RLU values to secreted levels of hGH, determined by radioimmunoassay. Error bars represent the standard error based on at least two independent experiments.