Several members of the Janus family of protein tyrosine kinases have been described. These include Tyk 2, JAK1, JAK2 and JAK3. Subsequent to activation, these kinases function to activate various members of the Stat family of transcriptional factors by phosphorylation on critical tyrosine regulatory sites. JAK3 (also designated L-JAK), is activated in response to IL-2 and IL-4 in T cells and myeloid cells. Expression of JAK3 is restricted to T cells, whereas the other members of the Janus family of protein tyrosine kinases are much more widely expressed.