Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Polycythemia Vera ?
    ### Response:
    Key Points
                    - Polycythemia vera is a disease in which too many red blood cells are made in the bone marrow.    - Symptoms of polycythemia vera include headaches and a feeling of fullness below the ribs on the left side.    - Special blood tests are used to diagnose polycythemia vera.
    