The Ph-negative myeloproliferative neoplasms (MPNs), polycythemia vera (PV), essential thrombocythemia (ET), primary or idiopathic myelofibrosis (PMF), and secondary myelofibrosis (MF, secondary to PV or ET), share a common etiology in hyperactivation of the kinase JAK2 in the hematopoietic stem and progenitor cell (HSPC) compartment of a malignant clone. The gene discussed is JAK2; the disease is primary myelofibrosis.