ABL1 and Chronic Eosinophilic Leukemia, Not Otherwise Specified: MPNs encompass seven clinicopathologic entities: chronic myeloid leukemia (CML), chronic neutrophilic leukemia (CNL), chronic eosinophilic leukemia not otherwise specified (CEL-NOS), myeloproliferative neoplasm unclassifiable (MPN-U), polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF), all of which are classified as classical BCR-ABL negative MPNs [1,2].