Illustration of a megakaryocyte (purple), a large cell in bone marrow responsible for the production of platelets, and a hematopoietic stem cell (pink). Myelofibrosis is a rare disease in which scar tissues replace the bone marrow, commonly associated with a mutation in JAK2, CALR, or MPL. The overproduction of megakaryocytes in the bone marrow is accompanied by an increase in their release of cytokines, which cause the development of connective tissue in the bone marrow. This decreases the amount of hematopoietic tissues and stem cells, reducing the ability to create new blood cells.