ABL1 and essential thrombocythemia: The World Health Organization (WHO) provides diagnostic criteria for BCR-ABL negative MPNs, including polycythemia vera (PV), essential thrombocythemia (ET), primary myelofibrosis (PMF), chronic eosinophilic leukemia (CEL), chronic neutrophilic leukemia (CNL), and myeloproliferative neoplasms, unclassifiable (MPN-U) (1).