The myeloproliferative neoplasms (MPNs), polycythemia vera (PV), essential thrombocytosis (ET), and primary myelofibrosis (PMF), are hematopoietic stem cell (HSC) disorders that express mutations activating JAK2 (1–4), the cognate tyrosine kinase for the type 1 homodimeric hematopoietic growth factor receptors for erythropoietin (EPO) and thrombopoietin (THPO) (5), and the granulocyte-colony stimulating factor receptor as well (6). The gene discussed is JAK2; the disease is acquired polycythemia vera.