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. Although myelofibrosis can occur at any age, it typically develops after the age of 50. In most cases, myelofibrosis gets progressively worse. Treatment is aimed at relieving signs and symptoms and may include medications, blood transfusions, chemotherapy, radiation therapy and surgery. Source: Genetic and Rare Diseases Information Center (GARD), supported by ORDR-NCATS and NHGRI.