Myeloproliferative Neoplasms (MPNs) are blood cancers that occur when the body makes too many white or red blood cells, or platelets. Myeloproliferative neoplasms (MPN) are a group of diseases that affect blood- cell formation. Myeloproliferative neoplasms (MPN) are cancers that start in the. In myeloproliferative neoplasms (MPN), the bone marrow makes too many of one or. Myelofibrosis (MF) causes fibrous material to build up inside the bones, preventing the body from producing blood cells effectively.