Since the presence of nasal polyps and elevated peripheral blood eosinophils have both been associated with increased levels of IL-5 and elevated T helper type-2 pathway activity [27–29], we hypothesise that the mepolizumab treatment effect was most notable in patients with these characteristics as a result of its specific binding to (and therefore inactivation of) IL-5 [30]. The gene discussed is IL5; the disease is nasal cavity polyp.