To elucidate further the use of IGF-1R by flow cytometry to diagnose other MPN diseases, 19 patients with essential thrombocythemia (ET), 28 myelofibrosis (MF) patients including 5 post-ET-MF, 5 post-PV-MF, and 18 primary myelofibrosis (PMF) patients were compared with PV patients and controls. Here, IGF1R is linked to essential thrombocythemia.