Assessment of MPN patients lacking this JAK2 mutation led to the identification of activating exon 12 JAK2 mutations in PV [13], as well as mutations in the thrombopoietin receptor (MPL) [14] and in the endoplasmic reticulum chaperone protein calreticulin (CALR) in ET and PMF [15]. The gene discussed is CALR; the disease is acquired polycythemia vera.