Primary myelofibrosis

Primary myelofibrosis is a condition characterized by the buildup of scar tissue (fibrosis) in the spongy tissue tissue inside the bone (bone marrow), the tissue that contains the stem cells that will produce blood cells. Because of the fibrosis, the bone marrow is unable to make enough normal blood cells. In myelofibrosis, the bone marrow is replaced by fibrous (scar) tissue.^[1][2]  When the bone marrow is scarred, it cannot make enough blood cells. This leads to anemia, weakness, fatigue, and often, swelling of the liver and spleen.^[1][3] The disorder occurs when blood stem cells develop somatic mutations in the JAK2, MPL, CALR, and TET2 genes. Other genes may also be involved. The disorder is generally not inherited because this type of mutation does not affect the reproductive cells (sperm and egg) only certain cells of the body (somatic).^[2] Although myelofibrosis can occur at any age, it typically develops after the age of 50 years.^[1][3] 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.^[3] Bone marrow or stem cell transplant may improve symptoms, and may cure the disease.^[1]   

Few epidemiologic studies are available to estimate the occurrence of myelofibrosis.^[4] Through the review of several medical textbooks and journal articles, it appears as if the annual incidence (occurrence) in European, Australian, and North American populations ranges from 0.3 to 1.5 cases per 100,000 persons.^{[4][5][6][7][8]}

According to the definition of a rare disorder per the Orphan Drug act, myelofibrosis is considered a rare disorder given the incidence discussed above.