Primary myelofibrosis is a relatively rare bone marrow cancer. It is currently classified as a myeloproliferative neoplasm, in which the proliferation of an abnormal clone of hematopoietic stem cells in the bone marrow and other sites results in fibrosis, or the replacement of the marrow with scar tissue.The term myelofibrosis alone usually refers to primary myelofibrosis (PMF), also known as chronic idiopathic myelofibrosis (cIMF); the terms idiopathic and primary mean that in these cases the disease is of unknown or spontaneous origin. This is in contrast with myelofibrosis that develops secondary to polycythemia vera or essential thrombocythaemia. Myelofibrosis is a form of myeloid metaplasia, which refers to a change in cell type in the blood-forming tissue of the bone marrow, and often the two terms are used synonymously. The terms agnogenic myeloid metaplasia and myelofibrosis with myeloid metaplasia (MMM) were also used to refer to primary myelofibrosis.