Primary myelofibrosis (PMF) is a rare bone marrow blood cancer. It is classified by the World Health Organization (WHO) as a type of myeloproliferative neoplasm, a group of cancers in which there is growth of abnormal cells in the bone marrow. This is most often associated with a somatic mutation in the JAK2, CALR, or MPL gene markers. In PMF, the healthy marrow is replaced by scar tissue (fibrosis), resulting in a lack of production of normal blood cells. Symptoms include anemia, increased infection and an enlarged spleen (splenomegaly). In 2016, prefibrotic primary myelofibrosis was formally classified as a distinct condition that progresses to overt PMF in many patients, the primary diagnostic difference being the grade of fibrosis. Signs and symptoms The primary feature of primary myelofibrosis is bone marrow fibrosis, but it is often accompanied by: Abdominal fullness related to an enlarged spleen (splenomegaly). Bone pain Bruising and easy bleeding due to inadequate numbers of platelets Cachexia (loss of appetite, weight loss, and fatigue) Enlargement of both the liver and spleen Fatigue Fevers Chills Weight loss Gout and high uric acid levels Increased susceptibility to infection, such as pneumonia Pallor and shortness of breath due to anemia In rarer cases, a raised red blood cell volume Cutaneous myelofibrosis is a rare skin condition characterized by dermal and subcutaneous nodules. Causes The underlying cause of PMF is unknown (idiopathic disease). There is an association between mutations to the JAK2, CALR, or MPL gene and myelofibrosis. Approximately 90% of those with myelofibrosis have one of these mutations and 10% carry none of these mutations. These mutations are not specific to myelofibrosis, and are linked to other myeloproliferative neoplasms, specifically polycythemia vera and essential thrombocythemia. The V617F mutation to the JAK2 protein is found in approximately half of individuals with primary myelofibrosis. The V617F mutation is a change of valine to phenylalanine at the 617 position. Janus kinases (JAKs) are non-receptor tyrosine kinases essential for the activation of signaling that is mediated by cytokine receptors lacking catalytic activity. These include receptors for erythropoietin, thrombopoietin, most interleukins and interferon. JAK2 mutations are significant because JAK2 plays a role in controlling production of blood cells from hematopoietic stem cells. The V617F mutation appears to make hematopoietic cells more sensitive to growth factors that need JAK2 for signal transduction, which include erythropoietin and thrombopoietin. The MPL gene codes for a protein that acts as a