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JAK2 V617F is an acquired (not inherited) change in the JAK2 gene in which phenylalanine replaces valine at position 617 of the JAK2 protein, leaving the enzyme constitutively active. It is found in about 95-97% of polycythemia vera (PV) and roughly half of essential thrombocythemia (ET) and primary myelofibrosis (PMF). It is also found in some healthy older people, so a positive result is not specific to a blood cancer. This page is general scientific information, not medical advice.
Short answer
V617F is the most common driver variant in the Philadelphia-negative myeloproliferative neoplasms (MPNs). It switches on the JAK/STAT signalling pathway that governs blood-cell production. Labs report it as present or absent and, in many assays, as a variant allele frequency (VAF). What the result means depends on the clinical context, which is why it is read alongside blood counts, bone marrow findings and other markers.
Definition and scope
JAK2 encodes Janus kinase 2, a protein that, per MedlinePlus Genetics (NIH/NLM), works in the JAK/STAT pathway and is important for producing blood cells from stem cells in bone marrow. The notation Val617Phe, or V617F, describes an amino acid swap at position 617: valine becomes phenylalanine (see gene notation conventions for how such labels are written). The change sits in the JH2 pseudokinase domain, which has no enzymatic activity of its own and normally restrains the kinase domain (Int J Mol Sci review, 2022). With the brake removed, the protein is constitutively activated.
JAK2 relays signals for receptors that lack their own kinase activity, including the erythropoietin (EPO), thrombopoietin (TPO) and G-CSF receptors. Increased signalling drives overproduction of red cells, platelets and neutrophils (Moliterno et al., Blood 2023). The variant is somatic: it arises during life in a subset of blood cells and is not passed on to children (MedlinePlus Genetics).
Which diseases carry it
| Condition | Reported share with JAK2 V617F | Source |
|---|---|---|
| Polycythemia vera (PV) | About 96% (MedlinePlus); 95% (IJMS 2022); 97% (Moliterno 2023) | All three |
| Essential thrombocythemia (ET) | About half (MedlinePlus); 50-60% (IJMS 2022) | MedlinePlus; IJMS 2022 |
| Primary myelofibrosis (PMF) | About half (MedlinePlus); 50-60% (IJMS 2022) | MedlinePlus; IJMS 2022 |
The figures differ slightly between sources because cohorts and assay sensitivity differ. Homozygosity for the variant is reported in roughly 25-30% of PV but only 2-4% of ET (IJMS 2022). MedlinePlus also notes occasional findings in leukemia and other marrow disorders, and in Budd-Chiari syndrome when an underlying marrow disorder is present.
How it is used in classification and testing
The 2005 discovery of the variant, Moliterno and colleagues write, formed the basis for disease classification criteria and targeted therapeutics. PV, ET and PMF were grouped as Philadelphia-negative MPNs in the WHO 2016 classification (IJMS 2022). CALR and MPL are the other principal driver genes. When PV is suspected but V617F is absent, the review advises looking for mutations in JAK2 exon 12, which MedlinePlus identifies as the change in a small number of patients.
Testing has become much more sensitive. Quantitative assays now detect a VAF below 1%, and one study used an assay sensitive to 0.009% VAF (Moliterno 2023). That makes the assay’s limit of detection a real design question; the general concept is explained in our limit of detection vs limit of quantitation guide. Allele burden, the VAF measured in a sample, is a separate quantity from whether the variant is present, and cells that are heterozygous and homozygous for V617F differ in signalling strength (Moliterno 2023).
What a positive result does and does not mean
- Acquired, not inherited. It is not a germline finding, so it is not a family-risk test (MedlinePlus Genetics).
- Not specific to MPN. In a Danish general-population study cited by Moliterno et al., JAK2 V617F-positive clonal hematopoiesis was detected in 3.1% of people, with a median VAF of 2.1%.
- Not present in every MPN. About half of ET and PMF cases lack it and may carry CALR or MPL changes instead (IJMS 2022).
- Absence in suspected PV prompts a search of exon 12 rather than ruling the disease out (IJMS 2022).
Related sensitivity concepts for interpreting any test are in sensitivity vs specificity.
Where it matters for labs and research
For molecular labs, the practical points are assay sensitivity (a detection threshold under 1% VAF changes how low-level clones are reported), whether the report gives a quantitative VAF, and whether reflex testing of CALR, MPL or JAK2 exon 12 is part of the workflow. For researchers, the variant is a standard marker for hematologic-malignancy cohorts and for studies of clonal hematopoiesis. For background see what is hematology; for how a clinical risk tier for ET thrombosis appears in a stratification model, see risk stratification. Variant interpretation frameworks are covered in the ACMG/AMP variant classification guide.
Common misunderstandings
- “V617F is the cause of all MPNs.” It is the most common driver, not the only one.
- “A positive result means cancer.” Clonal hematopoiesis with the variant occurs in the general population (3.1% in the Danish study).
- “It runs in families.” The variant is acquired in blood cells; it is not inherited.
- “V617F and allele burden are the same thing.” Presence is qualitative; VAF is a measured proportion.
FAQ
What does V617F mean?
Valine (V) at position 617 of the JAK2 protein is replaced by phenylalanine (F), written Val617Phe (MedlinePlus Genetics).
Which diseases are associated with JAK2 V617F?
Mainly polycythemia vera, essential thrombocythemia and primary myelofibrosis; see the table above for sourced frequencies.
Is JAK2 V617F inherited?
No. It is acquired during a person’s lifetime in certain cells (MedlinePlus Genetics).
What is the alternative if V617F is not found in suspected PV?
JAK2 exon 12 mutations are reported in a small number of patients and are the usual next target (MedlinePlus; IJMS 2022).
Sources
- MedlinePlus Genetics, JAK2 gene (NIH/NLM), read 2026-11-11.
- Moliterno AR, Kaizer H, Reeves BN. JAK2 V617F allele burden in polycythemia vera: burden of proof. Blood 2023;141(16):1934-1942. PMC10163319, read 2026-11-11.
- Review of JAK2 V617F biology and MPN, Int J Mol Sci 2022. PMC8835324, read 2026-11-11.
