IL21R,a receptor for interleukin-21,belongs to the type I cytokine receptor family. IL21R has been shown to form a heterodimeric receptor complex with the common gamma-chain,a receptor subunit also shared by the receptors for interleukin 2,4,7,9,and 15. This receptor transduces the growth promoting signal of IL21,and is important for the proliferation and differentiation of T cells,B cells,and natural killer (NK) cells. The ligand binding of this receptor leads to the activation of multiple downstream signaling molecules,including JAK1,JAK3,STAT1,and STAT3.