STAT5 activation by G-CSF-R–d715 is insensitive to inhibition by SOCS3, whereas STAT3 activation is not. HEK293 cells transfected with STAT5 or STAT3 luciferase reporter constructs were stimulated with G-CSF for 6 hours and assayed for luciferase activity. G-CSF–induced STAT5 (A) or STAT3 (B) luciferase reporter activity in the absence of SOCS3 was set at 100%. Data are expressed as mean + 95% confidence interval of 4 independent experiments. *Differences between WT and G-CSF-R–d715 are significant; P < .01.