Myelofibrosis with myeloi metaplasia, also known as agnogenic myeloid metaplasia, chronic idiopathic myelofibrosis, and primary myelofibrosis, was first described in 1879 and is currently classified as a myeloproliferative disease caused by the growth and proliferation of an abnormal bone marrow stem cell, resulting in the replacement of the bone marrow with fibrous connective tissue. An eponym for the disease is Heuck-Assmann disease, or Assmann's Disease.