Janus Kinase 2
**Note 1:** Physician statement of JAK2 can be used to code this data item when no other information is available.
**Note 2:** Janus Kinase 2 (JAK2, JAK 2) is a gene mutation that increases susceptibility to several myeloproliferative neoplasms (MPNs). Testing for the JAK2 mutation is done on whole blood. Nearly all people with polycythemia vera, and about half of those with primary myelofibrosis and essential thrombocythemia, have the mutation.
**Note 3:** Record JAK2 for any hematopoietic neoplasm. It is most commonly used for the following histologies:
* Polycythemia Vera (9950/3)
* Primary myelofibrosis (9961/3)
* Essential Thrombocytopenia (9962/3)
* Chronic myelomonocytic leukemia (9945/3)