IL-21 R, also called NILR (novel interleukin receptor) is a type I cytokine receptor with four conserved cysteine residues and an extracellular WSXWS motif. It is most closely related to IL-2 R beta and IL-9 R alpha. Mouse and human IL-21 R share 62% amino acid identity. IL-21 R is expressed on lymphoid tissues, peripheral B cells, and cell lines of T, B and natural killer cell lineage. IL-21 mediated signaling requires the common gamma chain in addition to IL-21 R.