Below is an instruction from Human. Write a response.
    ### Instruction:
    What causes Polycythemia Vera ?
    ### Response:
    Primary Polycythemia
                
Polycythemia vera (PV) also is known as primary polycythemia. A mutation, or change, in the body's JAK2 gene is the main cause of PV. The JAK2 gene makes a protein that helps the body produce blood cells.
                

    