Reduced internalization of truncated G-CSF-R mutants alleviates inhibition by SOCS3 only in the absence of Tyr729. Comparison of G-CSF-R mutants with normal (WT and Tyr729Phe) and defective (d715, d749-769, and d749-769Phe) internalization kinetics for sensitivity to SOCS3-mediated inhibition of STAT5 luciferase reporter activity. STAT5 luciferase reporter assay was performed as described in Figure 3. Data are expressed as mean of at least 2 independent experiments with triplicate measurements. *Difference between WT and d749-769 versus all other G-CSF-R mutants is significant, P < .01. #Difference between Tyr729Phe and d749-769, P = 0.07. ϕDifference between TyrY729Phe versus d715 and d749-769Phe is significant; P < .02.