Mepolizumab was administered to a handful of patients with HES in two short mono-centric open-label studies (40, 41), the clinical results of which were so encouraging that orphan drug status was granted, and an international placebo-controlled double-blind randomized clinical trial was undertaken to assess efficacy in this rare disease in 2004 (42) (see Clinical Trials Evaluating Antibodies That Target IL-5 or Its Receptor in Systemic HESs). The gene discussed is IL5; the disease is hypereosinophilic syndrome.