Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Myelofibrosis ?
    ### Response:
    Myelofibrosis is a disorder of the bone marrow, in which the marrow is replaced by fibrous (scar) tissue. Scarring of the bone marrow causes anemia, which can lead to fatigue and weakness, as well as pooling of the blood in abnormal sites like the liver and spleen, causing these organs to swell.
    