FIP1L1 and hypereosinophilic syndrome: In the United States, mepolizumab is currently the only approved therapy in this drug class for the treatment of HES without an identifiable nonhematologic secondary cause28, 29, 30; other anti–IL-5 and anti–IL-5 receptor biologics are also currently being studied for this population.31, 32, 33, 34 Imatinib is approved for Fip1-like 1 platelet-derived growth factor receptor mutated myeloid HES and chronic eosinophilic leukemia.35