BCR and Chronic Eosinophilic Leukemia, Not Otherwise Specified: Chronic eosinophilic leukemia (CEL; previously also known as chronic eosinophilic leukemia, not otherwise specified), is an infrequent and challenging subtype of BCR::ABL negative myeloproliferative neoplasm (MPN) that primarily targets eosinophils—a crucial component of the body's immune response and allergy regulation1–3.