Janus kinase 3 deficiency or JAK3 deficiency is a defect in the body's cytokine receptors and their signaling. [1] JAK3 encodes Janus kinase 3, a tyrosine kinase that belongs to the Janus family. JAK3 functions in signal transduction and interacts with members of the STAT (signal transduction and activators of transcription) family. The cause of JAK3 deficiency. [2] The deficiency causes the near absence of T lymphocytes and Natural killer cells; and normal or elevated B lymphocytes due to an autosomal recessive variant of severe combined immunodeficiency (SCID) [3]