Polycythemia vera is a condition characterized by an increased number of red blood cells in the bloodstream (erythrocytosis). Affected people may also have excess white blood cells and platelets. Conditions where the body makes too many of these cells are known as myeloproliferative neoplasms. These extra cells cause the blood to be thicker than normal, increasing the risk for blood clots that can block blood flow in arteries and veins. If a blood clot occurs in the veins deep in the arms and the legs, it is known as deep vein thrombosis (DVT). A DVT can sometimes travel through the blood stream to the lungs, which can cause a pulmonary embolism and is very dangerous. A blood clot could also travel to the heart or brain, which leads to an increased risk for heart attack or stroke. Most cases of PV are not inherited and are acquired during a person's lifetime. PV occurs more frequently in men than it does in women. The condition has been associated with genetic changes in the JAK2 and TET2 genes. In rare cases, the risk for PV runs in families and may be inherited in an autosomal dominant manner.