Janus kinases (JAKs), belonging to a distinct family of tyrosine kinases, are non-receptor tyrosine kinases that transduce signals of cytokine receptors via diverse signal transduction pathways. Mammalian JAK family consists of four structurally similar members, JAK1, JAK2, JAK3 and Tyrosine kinase 2 (TYK2), which are characterized by containing seven JAK homology (JH) domains. The presence of JH1 and JH2 domains is the most intriguing feature of JAK proteins. JH1 domain is the main catalytic domain; while JH2 is usually considered as a catalytically inactive pseudokinase domain. However, results of recent studies have shown that JH2 acts as a dual-specificity protein kinase to phosphorylate two regulatory sites (Ser523 and Tyr570) in JAK2.