Since the first two heterozygous MPL point mutations were identified in congenital amegakaryocytic thrombocytopenia (CAMT) in 1999, mutations in hereditary thrombocythaemia (HT), myeloproliferative neoplasms (MPNs), refractory anaemia with ringed sideroblasts associated with marked thrombocytosis (RARS-t) and acute myeloid leukaemia (AML) have been identified [22]. The gene discussed is MPL; the disease is hematocrit.