In the 2017 World Health Organization (WHO) classification, chronic eosinophilic leukemia, not otherwise specified (CEL, NOS), is included as one of 7 distinct diagnostic entities under the major category of myeloproliferative neoplasms (MPNs).1 WHO defining criteria for CEL, NOS include: (a) peripheral blood eosinophilia >1.5×109/L; (b) evidence of clonal cytogenetic or molecular genetic abnormality, and/or (c) presence of excess myeloblasts (<20%) in the peripheral blood (≥2%) or bone marrow (≥5%). In addition, other eosinophilia-associated hematolymphoid neoplasms require exclusion, particularly myeloid malignancies associated with eosinophilia such as acute myeloid leukemia with inv(16)(p13.1q22) or t(16;16)(p13.1;q22) CBFB-MYH11, MPNs including chronic myeloid leukemia, myelodysplastic syndromes MDS, MDS/MPNs, and myeloid/lymphoid neoplasms with eosinophilia and fusion genes involving tyrosine kinases (e.g., PDGFRA, PDGFRB, FGFR1, or PCM1-JAK2; see SOHO abstract by Reiter et al). CEL, NOS is a very rare neoplasm among patients presenting with eosinophilia. For example, in a Mayo Clinic series of 1416 patients with peripheral blood eosinophilia who were evaluated between 2008 and 2019, only 17 patients (1.2%) fulfilled the WHO diagnostic criteria for CEL, NOS.2 An analysis of the Surveillance, Epidemiology, and End Results (SEER) data between 2004 and 2015 revealed a stable incidence rate of 0.4 cases/1,000,000 persons.3