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 signalling pathways and is essential for the activation of immune and blood-forming (haematopoietic) cells. Mutations in JAK2 are linked to blood disorders, including polycythaemia vera and other myeloproliferative neoplasms.