The one disease variant for which IL-5 targeting should not be considered an option is chronic eosinophilic leukemia (CEL) with well-documented underlying cytogenetic rearrangements, most commonly the FIP1L1/PDGFRA (F/P) fusion gene (68). This evidence concerns the gene PDGFRA and Chronic Eosinophilic Leukemia, Not Otherwise Specified.