Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) primary myelofibrosis ?
    ### Response:
    Primary myelofibrosis is a condition characterized by the buildup of scar tissue (fibrosis) in the bone marrow, the tissue that produces blood cells. Because of the fibrosis, the bone marrow is unable to make enough normal blood cells. The shortage of blood cells causes many of the signs and symptoms of primary myelofibrosis.
    