Although there is still some uncertainty as to the value of using a single test to characterize the severity of eosinophilic inflammation [32–34], patients with blood eosinophil counts ≥ 300 cells/μL benefit from anti–IL-5 and anti–IL-5 receptor α therapy for reducing steroid use and exacerbations and improving control of asthma symptoms [16, 17, 35–37]. This evidence concerns the gene IL5 and asthma.