A gene that makes a protein called JAK2, which sends signals in cells that promote cell growth. This protein helps control the number of red blood cells, white blood cells, and platelets that are made in the bone marrow. Mutated (changed) forms of the JAK gene have been found in some types of blood conditions, including polycythemia vera, essential thrombocythemia, and primary myelofibrosis. These changes may cause the body to make too many blood cells.