Reviewed December 2011
What is the official name of the JAK2 gene?
The official name of this gene is “Janus kinase 2.”
JAK2 is the gene's official symbol. The JAK2 gene is also known by other names, listed below.
Read more about gene names and symbols on the About page.
What is the normal function of the JAK2 gene?
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. These stem cells are located within the bone marrow and have the potential to develop into red blood cells, white blood cells, and platelets.
How are changes in the JAK2 gene related to health conditions?
essential thrombocythemia - associated with the JAK2 gene
Some gene mutations are acquired during a person's lifetime and are present only in certain cells. These changes, which are called somatic mutations, are not inherited. Somatic mutations in the JAK2 gene are associated with essential thrombocythemia, a disorder characterized by an increased number of platelets, the blood cells involved in normal blood clotting. The most common mutation (written as Val617Phe or V617F) replaces the protein building block (amino acid) valine with the amino acid phenylalanine at position 617 in the protein. This particular mutation is found in approximately half of people with essential thrombocythemia. A small number of affected individuals have a somatic mutation in another part of the JAK2 gene known as exon 12.
The V617F JAK2 gene mutation results in the production of a JAK2 protein that is constantly turned on (constitutively activated), which, in essential thrombocythemia, leads to the overproduction of abnormal blood cells called megakaryocytes. Because platelets are formed from megakaryocytes, the overproduction of megakaryocytes results in an increased number of platelets. Excess platelets can cause abnormal blood clotting (thrombosis), which leads to many signs and symptoms of essential thrombocythemia.
polycythemia vera - associated with the JAK2 gene
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 polycythemia vera. About 3 percent of affected individuals have a somatic mutation in the exon 12 region of the JAK2 gene.
JAK2 gene mutations result in the production of a constitutively activated JAK2 protein, which seems to improve the survival of the cell and increase production of blood cells. With so many extra cells in the bloodstream, abnormal blood clots are more likely to form. In addition, the thicker blood flows more slowly throughout the body, which prevents organs from receiving enough oxygen. Many of the signs and symptoms of polycythemia vera are related to a lack of oxygen in body tissues.
primary myelofibrosis - associated with the JAK2 gene
Somatic JAK2 gene mutations are also 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 primary myelofibrosis. A small number of people with this condition have mutations in the exon 12 region of the gene. These JAK2 gene mutations result in a constitutively active JAK2 protein, which leads to the overproduction of abnormal megakaryocytes. These megakaryocytes stimulate other cells to release collagen, a protein that normally provides structural support for the cells in the bone marrow but causes scar tissue formation in primary myelofibrosis. Because of the fibrosis, the bone marrow cannot produce enough normal blood cells, leading to the signs and symptoms of the condition.
- other disorders - associated with the JAK2 gene
Somatic JAK2 gene mutations are also associated with several related conditions. The V617F mutation is occasionally found in people with cancer of blood-forming cells (leukemia) or other bone marrow disorders. Budd-Chiari syndrome, which results from a blocked vein in the liver, can also be associated with the V617F mutation when it is caused by an underlying bone marrow disorder. It is unknown how one particular mutation can be associated with several conditions.
Where is the JAK2 gene located?
Cytogenetic Location: 9p24
Molecular Location on chromosome 9: base pairs 4,985,244 to 5,128,182
The JAK2 gene is located on the short (p) arm of chromosome 9 at position 24.
More precisely, the JAK2 gene is located from base pair 4,985,244 to base pair 5,128,182 on chromosome 9.
See How do geneticists indicate the location of a gene? in the Handbook.
Where can I find additional information about JAK2?
You and your healthcare professional may find the following resources about JAK2 helpful.
You may also be interested in these resources, which are designed for genetics professionals and researchers.
What other names do people use for the JAK2 gene or gene products?
- Janus kinase 2 (a protein tyrosine kinase)
- tyrosine-protein kinase JAK2
Where can I find general information about genes?
The Handbook provides basic information about genetics in clear language.
These links provide additional genetics resources that may be useful.
What glossary definitions help with understanding JAK2?
The resources on this site should not be used as a substitute for
professional medical care or advice. Users seeking information about
a personal genetic disease, syndrome, or condition should consult with a qualified
See How can I find a genetics professional in my area? in the Handbook.