JAK2 (Janus Kinase 2): JAK2 is a gene that encodes a kinase enzyme belonging to the Janus kinase (JAK) family. JAK2 plays a key role in cytokine signaling pathways and is essential for the activation of immune and blood-forming (hematopoietic) cells. Mutations in JAK2 are linked to blood disorders, including polycythemia vera and other myeloproliferative neoplasms.