Polycythemia vera (PV), essential thrombocythemia (ET), and myelofibrosis (MF) including pre-fibrotic/early primary myelofibrosis (PMF) are classical Philadelphia chromosome-negative myeloproliferative neoplasms (MPNs) characterized by the uncontrolled clonal proliferation of hematopoietic stem or progenitor cells due to driver mutations of genes including JAK2, CALR, or MPL (1, 2). Here, JAK2 is linked to essential thrombocythemia.