Tyrosine-protein kinase JAK3 is a tyrosine kinase enzyme that in humans is encoded by the JAK3 gene. As JAK3 is expressed in hematopoietic and epithelial cells, its role in cytokine signaling is thought to be more restricted than other JAKs. It is most commonly expressed in T cells and NK cells, but has also been found in intestinal epithelial cells. JAK3 is involved in signal transduction by receptors that employ the common gamma chain (γc) of the type I cytokine receptor family (e.g. IL-2R, IL-4R, IL-7R, IL-9R, IL-15R, and IL-21R). Mutations that abrogate Janus kinase 3 function cause an autosomal SCID (severe combined immunodeficiency disease), while activating Janus kinase 3 mutations lead to the development of leukemia. In addition to its well-known roles in T cells and NK cells, JAK3 has been found to mediate IL-8 stimulation in human neutrophils. IL-8 primarily functions to induce chemotaxis in neutrophils and lymphocytes, and JAK3 silencing severely inhibits IL-8-mediated chemotaxis.