Polycythemia vera (PV) is primarily classified as a blood disorder, but it has characteristics of a bone marrow cancer. It's a type of myeloproliferative neoplasm, which means it involves the overproduction of blood cells due to abnormalities in the bone marrow. In PV, there is an excessive production of red blood cells, though white blood cells and platelets can also be increased. The disorder is often driven by a mutation in the JAK2 gene, which affects the regulation of blood cell production. So, while it originates from a problem in the bone marrow, it manifests primarily as a disorder of the blood. So, technically it can be described as either a blood disorder or a type of bone marrow cancer.