Mepolizumab reduces the occurrence of flares in hypereosinophilic syndrome (HES) and was approved by the Food and Drug Administration for the treatment of eosinophilic granulomatosis with polyangiitis (EGPA) in 2017, particularly in asthmatic patients.87,88 In a phase 2 trial involving symptomatic patients with FIP1L1-negative and PDGRFA-negative hypereosinophilia, benralizumab was associated with lower absolute eosinophil counts and long-term sustained response as compared with placebo.89,90. This evidence concerns the gene FIP1L1 and hypereosinophilic syndrome.