While a complete loss of TPO-R leads to thrombocytopenia, other mutations in MPL have been identified in patients with increased platelet numbers, in disorders such as familial thrombocytosis and two of the myeloproliferative neoplasms (MPNs), essential thrombocythemia (ET), and myelofibrosis (MF) (28–35). The gene discussed is MPL; the disease is familial thrombocytosis.