Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Primary Myelofibrosis ?
    ### Response:
    Key Points
                    - Primary myelofibrosis is a disease in which abnormal blood cells and fibers build up inside the bone marrow.    - Symptoms of primary myelofibrosis include pain below the ribs on the left side and feeling very tired.    - Certain factors affect prognosis (chance of recovery) and treatment options for primary myelofibrosis.
    