The coexistence of T-cell clonality and the F/P fusion gene in 5%–28% of CEL patients may provide insight into the complex pathogenesis, but also indicates that IL-5 may be the most relevant cytokine in the pathogenesis of F/P-mediated CEL [35], [36]. Here, IL5 is linked to Chronic Eosinophilic Leukemia, Not Otherwise Specified.