Many cytokines function through interaction with receptors of the cytokine 
receptor superfamily. Although lacking catalytic domains, cytokine receptors 
couple ligand binding to induction of protein tyrosine phosphorylation. Recent 
studies have shown that one or more of the Janus kinase family members (Jaks) 
associate with cytokine receptors and are tyrosine phosphorylated and activated 
following ligand binding. Here we describe a new Jak family kinase, Jak-3, and 
demonstrate that Jak-3, and to a lesser extent Jak-1, are tyrosine 
phosphorylated and Jak-3 is activated in the responses to interleukin-2 and 
interleukin-4 in T cells and myeloid cells. Jak-3 activation requires the 
serine-rich, membrane-proximal domain of the interleukin-2 receptor beta-chain, 
but does not require the acidic domain that is required for association and 
activation of Src family kinases.