 Moreover, unlike other JAKs, JAK3 is primarily expressed in hematopoietic cells, so a highly specific JAK3 inhibitor should have precise effects on immune cells and minimal pleiotropic defects.The selectivity of a JAK3 inhibitor would also have advantages over the current widely used immunosuppressant drugs, which have abundant targets and diverse side effects.A JAK3 inhibitor could be useful for treating autoimmune diseases, especially those in which a particular cytokine receptor has a direct role on disease pathogenesis.