What is the definition or description of: jak2 mutation?

See below: The jak2 mutation is a genetic change found in about 9 out of 10 people with polycythemia vera (pv). Jak2 is part of a signaling system (like a thermostat) that helps tell the bone marrow when to start and stop making blood cells. Most people with pv have acquired a mutation(change) in their jak2. Because of this change, the bone marrow makes too many blood cells.