IL-21R tyrosine mutants and their expression on Ba/F3 transfectants. (A) Sequences spanning the 6 tyrosines in the mouse IL-21R cytoplasmic domain. A consensus Stat3-binding motif (YLRQ) is boxed. (B) Schematic of the cytoplasmic domain of WT IL-21R and IL-21R mutants. TM indicates transmembrane region; Y and F, tyrosine and phenylalanine, respectively. (C) WT and mutant IL-21R constructs cloned into pRV-IRES-GFP vector were transfected stably in Ba/F3 cells. The expression of IL-21R was evaluated by examining the coexpressed GFP on a FACSort.