IL-21 is a novel cytokine that was recently identified in human and mouse. It has significant homology to IL-2, IL-4, and IL-15. The receptor for IL-21 (IL-21R, also termed NILR for novel interleukin receptor) is a new member of the class I cytokine receptor family. IL-021R forms a complex with the common cytokine receptor g chain and mediates IL-21 signaling. IL-21 and its receptor activates JAK-STAT signaling pathway. IL-21 is expressed in activated T cells, HL-60, and THP-1 cell lines. IL-21 plays an important role in the proliferation and maturation of NK, B and T cell populations.