Polycythemia vera (PV) is a rare blood disorder characterized by the overproduction of red blood cells by the bone marrow. This leads to an increase in blood volume and viscosity, causing a range of symptoms and complications. PV is classified as a myeloproliferative disorder and is often caused by a mutation in the JAK2 gene. In addition to red blood cells, there may also be an overproduction of white blood cells and platelets.