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JAK2 gene
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Gene

JAK2 gene

The JAK2 gene provides instructions for making a protein that promotes the growth and division of cells, particularly blood cells.

Normal Function

The JAK2 gene provides instructions for making a protein that promotes the growth and division (proliferation) of cells. This protein is part of a signaling pathway called the JAK/STAT pathway, which transmits chemical signals from outside the cell to the cell's nucleus. The JAK2 protein is especially important for controlling the production of blood cells from hematopoietic stem cells located within the bone marrow, which have the potential to develop into red blood cells, white blood cells, and platelets.

Health Conditions Related to Genetic Changes

Essential thrombocythemia

Somatic mutations in the JAK2 gene are associated with essential thrombocythemia, a disorder characterized by an increased number of platelets. The most common mutation (Val617Phe or V617F) replaces the amino acid valine with phenylalanine at position 617 in the protein. This mutation is found in approximately half of people with essential thrombocythemia.

The V617F mutation results in a JAK2 protein that is constantly turned on (constitutively activated), leading to the overproduction of abnormal blood cells called megakaryocytes. Because platelets are formed from megakaryocytes, this results in an increased number of platelets, which can cause abnormal blood clotting (thrombosis).

Polycythemia vera

Somatic mutations in the JAK2 gene are associated with polycythemia vera, a disorder characterized by uncontrolled blood cell production. The V617F mutation is found in approximately 96 percent of people with this condition. These mutations result in a constitutively activated JAK2 protein, which improves cell survival and increases blood cell production. This leads to thicker blood that flows more slowly, increasing the risk of abnormal blood clots and preventing organs from receiving enough oxygen.

Primary myelofibrosis

Somatic JAK2 gene mutations are associated with primary myelofibrosis, a condition in which bone marrow is replaced by scar tissue (fibrosis). The V617F mutation is found in approximately half of individuals with this condition. The constitutively active JAK2 protein leads to the overproduction of abnormal megakaryocytes, which stimulate other cells to release collagen. This causes scar tissue formation, preventing the bone marrow from producing enough normal blood cells.

Other disorders

Somatic JAK2 gene mutations are also associated with several related conditions, including:

  • Cancer of blood-forming cells (leukemia)
  • Other bone marrow disorders
  • Budd-Chiari syndrome (a blocked vein in the liver)

Other Names for This Gene

  • JAK-2
  • JAK2_HUMAN
  • Janus kinase 2 (a protein tyrosine kinase)
  • JTK10
  • tyrosine-protein kinase JAK2

Frequently Asked Questions

Pregnancy in women with JAK2‑mutated myeloproliferative neoplasms is higher‑risk for thrombosis and miscarriage. Close hematology‑obstetric collaboration is essential. Low‑dose aspirin is usually continued, and if platelet counts are high or you have a prior thrombotic event, prophylactic low‑molecular‑weight heparin may be added. Cytoreductive agents such as interferon‑α are considered safe in pregnancy, whereas hydroxyurea is avoided.
JAK2‑mutated essential thrombocythemia carries a higher thrombotic risk than JAK2‑negative disease, especially if platelet count exceeds 1 000 × 10⁹/L, you have cardiovascular risk factors, or you are over 60. Low‑dose aspirin (unless contraindicated) and cytoreductive therapy (hydroxyurea or interferon‑α) are commonly used to keep platelet counts < 450 × 10⁹/L and reduce clot risk. Lifestyle measures—smoking cessation, blood pressure control, and regular exercise—also help.
The V617F mutation is present in about 96 % of polycythemia vera (PV) cases, but a diagnosis also requires clinical criteria such as elevated red cell mass, hemoglobin/hematocrit above threshold, and low serum erythropoietin. If you have the mutation but normal blood counts, you may have a clonal hematopoiesis of indeterminate potential (CHIP) and will need periodic monitoring rather than immediate PV treatment.
The mutation is identified using a quantitative PCR or allele‑specific assay on peripheral blood DNA, reporting the mutant allele burden (percentage of mutated JAK2). Baseline allele burden helps confirm diagnosis, but routine repeat testing is not required for disease monitoring; instead, clinicians track blood counts, symptoms, and imaging. However, a repeat test may be ordered if there is a sudden change in clinical status or to assess response to targeted therapies in clinical trials.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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