Interleukin 2 (IL-2) is a growth factor that is important for proliferation and homeostasis of lymphocytes. The IL-2 receptor (IL-2R) contains the following three subunits: the α chain (IL-2R ), which is expressed only on activated lymphocytes; theβ chain (IL-2R ); and the γ common chain (IL-2Rγ c), a subunit shared by several cytokines. The cytoplasmic domains of IL-2R subunits do not possess intrinsic enzymatic activity; rather, IL-2 binding induces oligomerization of IL-2R, which initiates activation of several protein tyrosine kinases (PTKs) and subsequent phosphorylation of the IL-2R complex.The Janus kinases (JAKS) family of PTKs play a central role in IL-2 signaling. JAK3 constitutively binds the cytoplasmic domain of IL-2Rγ c as well as IL-2Rβ , while JAK1 constitutively interacts with IL-2Rβ...