What you carry at this position in your DNA and what research says about it.
Blood Disorder Risk: 10/10 evidence, 11 publications.
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Sample genotype (not yours): G/G
Typical Blood Disorder Risk
The typical genotype is associated with normal blood cell production and baseline risk. There is no elevated risk of developing myeloproliferative neoplasms from this variant.
What this genotype means: Normal blood cell counts; Typical risk of myeloproliferative neoplasms.
Sample genotype (not yours): G/T
Elevated Blood Disorder Risk
Presence of one T allele is strongly linked to overproduction of blood cells and myeloproliferative disorders. This variant is typically acquired during life rather than inherited.
What this genotype means: Elevated platelet and red blood cell counts; Increased risk of blood clots and myeloproliferative neoplasms.
Sample genotype (not yours): T/T
High Blood Disorder Risk
Presence of two T alleles is associated with severe overproduction of blood cells and myeloproliferative neoplasms. This genotype typically arises from clonal expansion of mutated blood cells.
What this genotype means: Severely elevated blood cell counts; High risk of thrombotic complications and myelofibrosis.
Gene function and biological role
Gene Function: The JAK2 gene encodes the Janus kinase 2 protein, a non-receptor tyrosine kinase essential for cell growth and division [1]. It plays a critical role in controlling the production of blood cells from hematopoietic stem cells in the bone marrow [2]. Signals from cytokines and growth factors activate this protein to promote cell survival and proliferation [1]. Abnormalities in this signaling pathway can lead to the overproduction of red blood cells, white blood cells, and platelets [2]. This gene is highly studied due to its central involvement in chronic blood disorders and bone marrow cancers [1].
Variant Impact: The rs77375493 variant causes a missense change from valine to phenylalanine at position 617 of the JAK2 protein, commonly known as V617F [1]. This alteration occurs in the pseudokinase domain, which normally acts as a brake to keep the kinase activity in check [1]. The change disrupts this regulatory mechanism, leading to a gain-of-function where the protein is constantly active and signaling [3]. Although usually acquired as a somatic mutation in blood cells over a person's lifetime, it is highly associated with myeloproliferative neoplasms [1]. These include conditions like polycythemia vera, essential thrombocythemia, and primary myelofibrosis [4].
Key Insight: This variant is typically somatic, meaning it is acquired during life rather than inherited from parents, and is a hallmark of clonal hematopoiesis.
Location: JAK2. Chromosome 9 · 9p24.1 · 5.1 Mb
Estimated prevalence by population
Global 99.9%; European 99.9%; African 99.9%; East Asian > 99.9%; South Asian 99.9%; Americas 100.0%; Ashkenazi 99.8%; Finnish 99.9%; Middle Eastern 100.0%; Amish 100.0%.
Estimated GG genotype frequency.
Global 0.1%; European 0.1%; African 0.1%; East Asian < 0.1%; South Asian 0.1%; Americas 0.0%; Ashkenazi 0.2%; Finnish 0.1%; Middle Eastern 0.0%; Amish 0.0%.
Estimated GT genotype frequency.
Global < 0.1%; European < 0.1%; African < 0.1%; East Asian < 0.1%; South Asian < 0.1%; Americas 0.0%; Ashkenazi < 0.1%; Finnish < 0.1%; Middle Eastern 0.0%; Amish 0.0%.
Estimated TT genotype frequency.
Showing effects for G/G genotype
Cancer & Oncology
Blood Cancers
Myeloproliferative Disorders. Evidence: 10/10. Source: PMID 15781101. Typical Risk.
Cardiovascular & Metabolic
Blood Cell & Iron Traits
Platelet Count. Evidence: 10/10. Source: PMID 24777453. Typical.
Red Blood Cell Count. Evidence: 8/10. Source: PMID 39024449. Typical.
Lipids & Cholesterol
Cholesterol Levels. Evidence: 10/10. Source: PMID 30275531. Slightly Higher.
Showing effects for G/T genotype
Myeloproliferative Disorders. Evidence: 10/10. Source: PMID 15781101. Increased Risk.
Platelet Count. Evidence: 10/10. Source: PMID 24777453. Increased.
Red Blood Cell Count. Evidence: 8/10. Source: PMID 39024449. Increased.
Cholesterol Levels. Evidence: 10/10. Source: PMID 30275531. Slightly Lower.
Showing effects for T/T genotype
What this genotype means for the people who carry it
Sample genotype: GG (Homozygous)
Maintaining two copies of the reference G allele ensures normal regulation of the Janus kinase 2 protein [1]. Under normal conditions, blood cell production remains balanced and responds appropriately to physiological signals [2]. This genotype is found in the vast majority of the global population and represents the baseline risk for blood disorders [1]. Large-scale genetic studies confirm that individuals with this genotype do not experience the elevated platelet or red blood cell counts associated with the variant allele [3]. It is also associated with typical cholesterol levels, showing no genetic reduction in circulating lipids [4].
Normal blood cell counts; Typical risk of myeloproliferative neoplasms.
Sample genotype: GT (Heterozygous)
Presence of a single T allele leads to a hyperactive JAK2 protein that continuously signals for cell division [1]. This gain-of-function state is a primary driver of myeloproliferative neoplasms, including polycythemia vera and essential thrombocythemia [2]. Clinical studies show that even a single copy of this variant significantly increases platelet, red blood cell, and white blood cell counts [3]. It is also associated with a higher risk of blood clots, such as Budd-Chiari syndrome [4]. Because this variant is usually somatic, its detection in blood tests often indicates clonal hematopoiesis or an underlying blood disorder [1].
Elevated platelet and red blood cell counts; Increased risk of blood clots and myeloproliferative neoplasms.
Sample genotype: TT (Homozygous)
A homozygous state for the T allele represents a condition of complete JAK2 activation, which is highly characteristic of advanced polycythemia vera [1]. This genotype typically occurs when a somatic mutation undergoes mitotic recombination, leading to uniparental disomy in mutated blood cell clones [2]. The complete absence of the wild-type G allele results in uncontrolled erythropoiesis and megakaryopoiesis, leading to extremely high red blood cell and platelet counts [3]. Clinical literature associates this state with a high risk of cardiovascular and thrombotic complications [4]. It is also linked to a higher likelihood of progression to myelofibrosis or acute myeloid leukemia [1].
Severely elevated blood cell counts; High risk of thrombotic complications and myelofibrosis.
Variants with shared genetic architecture
rs10758669 · JAK2 Gene · Chr 9 · Inflammatory Bowel Disease Risk
rs385893 · RCL1 Gene · Chr 9 · Blood Cell Production
rs11075995 · FTO Gene · Chr 16 · Breast Cancer Risk
A gain-of-function mutation of JAK2 in myeloproliferative disorders.
A unique clonal JAK2 mutation leading to constitutive signalling causes polycythaemia vera.
Acquired mutation of the tyrosine kinase JAK2 in human myeloproliferative disorders.
Genetics of blood lipids among ~300,000 multi-ethnic participants of the Million Veteran Program.
Content updated August 2026.
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