The Janus kinases (Jaks) are a subgroup of non-receptor protein tyrosine kinases (PTKs) that play critical roles in immune cell signaling. The four Jaks—Jak1, Jak2, Jak3, and Tyk2—are essential for cell growth, survival, development, and differentiation, particularly in immune and hematopoietic cells. Deficiencies in Jak3 or Tyk2 lead to severe immunodeficiencies, such as severe combined immunodeficiency (SCID) and autosomal recessive hyperimmunoglobulin E syndrome (HIES). In contrast, complete deletion of Jak1 or Jak2 is not viable in mice and does not have human counterparts. However, activating mutations in Jaks, such as the V617F mutation in Jak2, are associated with myeloproliferative disorders like polycythemia vera. Understanding Jak signaling pathways has led to the development of selective Jak inhibitors, now in clinical trials for autoimmune diseases and malignancies. Jak signaling is primarily mediated through the STAT family of proteins, which are activated by phosphorylation. Jak3 is crucial for signaling through the γc chain of cytokine receptors, and its deficiency leads to SCID. Tyk2 is essential for signaling through type I IFNs and cytokines like IL-12 and IL-23, and its deficiency results in impaired Th1 and Th17 responses, increased Th2 responses, and susceptibility to infections. Jak2 is involved in erythropoiesis and myeloproliferative disorders, with mutations like V617F causing constitutive activation. Jak2 mutations are also linked to various cancers, including leukemia and myelofibrosis. Jak inhibitors, such as CP-690,550, are being tested for their efficacy in treating autoimmune diseases and preventing transplant rejection. These inhibitors target Jak3, which is highly expressed in hematopoietic cells, and have shown promise in preclinical and clinical trials for conditions like rheumatoid arthritis, psoriasis, and renal transplantation. Despite their potential, challenges remain in understanding the complex regulatory mechanisms of Jak signaling and the long-term effects of Jak inhibition. Continued research is essential to fully harness the therapeutic potential of Jak inhibitors in treating immune-related diseases and malignancies.The Janus kinases (Jaks) are a subgroup of non-receptor protein tyrosine kinases (PTKs) that play critical roles in immune cell signaling. The four Jaks—Jak1, Jak2, Jak3, and Tyk2—are essential for cell growth, survival, development, and differentiation, particularly in immune and hematopoietic cells. Deficiencies in Jak3 or Tyk2 lead to severe immunodeficiencies, such as severe combined immunodeficiency (SCID) and autosomal recessive hyperimmunoglobulin E syndrome (HIES). In contrast, complete deletion of Jak1 or Jak2 is not viable in mice and does not have human counterparts. However, activating mutations in Jaks, such as the V617F mutation in Jak2, are associated with myeloproliferative disorders like polycythemia vera. Understanding Jak signaling pathways has led to the development of selective Jak inhibitors, now in clinical trials for autoimmune diseases and malignancies. Jak signaling is primarily mediated through the STAT family of proteins, which are activated by phosphorylation. Jak3 is crucial for signaling through the γc chain of cytokine receptors, and its deficiency leads to SCID. Tyk2 is essential for signaling through type I IFNs and cytokines like IL-12 and IL-23, and its deficiency results in impaired Th1 and Th17 responses, increased Th2 responses, and susceptibility to infections. Jak2 is involved in erythropoiesis and myeloproliferative disorders, with mutations like V617F causing constitutive activation. Jak2 mutations are also linked to various cancers, including leukemia and myelofibrosis. Jak inhibitors, such as CP-690,550, are being tested for their efficacy in treating autoimmune diseases and preventing transplant rejection. These inhibitors target Jak3, which is highly expressed in hematopoietic cells, and have shown promise in preclinical and clinical trials for conditions like rheumatoid arthritis, psoriasis, and renal transplantation. Despite their potential, challenges remain in understanding the complex regulatory mechanisms of Jak signaling and the long-term effects of Jak inhibition. Continued research is essential to fully harness the therapeutic potential of Jak inhibitors in treating immune-related diseases and malignancies.