Essential thrombocythaemia (ET; also known as essential thrombocytosis, essential thrombocythemia, primary thrombocytosis) is a rare chronic blood disorder characterised by the overproduction of platelets by megakaryocytes in the bone marrow. It may, albeit rarely, develop into acute myeloid leukaemia or myelofibrosis. It is one of four myeloproliferative disorders (disorders characterised by increased production of a particular line of blood cell).