Together with polycythaemia vera (PV) and primary myelofibrosis (PMF), ET is one of the three myeloproliferative neoplasms (MPNs) characterised by stem-cell-derived clonal myeloproliferation with mutually exclusive Janus kinase (JAK)2V617F, calreticulin (CALR) or myeloproliferative leukaemia (MPL) mutations [6]. Here, CALR is linked to acquired polycythemia vera.