Hereditary thrombophilic states in patients with PV and ET, including factor V Leiden, prothrombin G20210A, and methylenetetrahydrofolate reductase (MTHFR) mutations, have been studied extensively [22,23); however, other polymorphisms, including PAI-1 4G/5G and ACE I/D gene mutations, haven’t been investigated in patients with PV and ET. The gene discussed is MTHFR; the disease is essential thrombocythemia.