JAK3 (Janus kinase 3) deficiency is a dysfunction in cytokine receptor signalling and their production of cytokines. JAK3 is a tyrosine protein kinase, an enzyme that is encoded by the JAK3 gene. It is a kinase that is activated only by cytokines whose receptors contain the common gamma chain subunit (IL-2, IL-4, IL-7, IL-9, IL-15 and IL-21). JAK3 is involved in iniating signalling for the cytokine receptors because they itself lack enzymatic activity. Once activated, the JAK kinase phosphorylates specific tyrosine residues on the cytokine receptor subunits which then activate STAT transcription factors. JAK3 helps to regulate differentiation and maturation of B cells, T cells and NK cells.