The JAK family of non-receptor intracellular tyrosine kinases is comprised of four members, JAK1, JAK2, JAK3 and tyrosine kinase (TyK)2. The JAKs are selectively activated by different receptors and have, therefore, distinct in vivo roles. JAK1 is mainly activated by type II cytokine receptors. JAK2 is crucial in transducing signals for cytokine receptors involved in hematopoiesis (erythropoietin, thrombopoietin and haematopoietic cell development cytokines). JAK3 is mainly expressed in B and T lymphocytes, and TYK2 associates commonly with other JAKs.