The mutational landscape of MPNs is formally integrated into the WHO diagnostic criteria for PV (98% JAK2 mutational frequency), ET (50–60% JAK2, 22% calreticulin (CALR), and 3% thrombopoietin receptor (myeloproliferative leukemia virus, MPL), and PMF (50–60% JAK2, 25% CALR, and 7% MPL). The gene discussed is MPL; the disease is acquired polycythemia vera.