While 300 mg mepolizumab has been approved for patients with idiopathic HES and has shown efficacy in decreasing the risk of flares as well as a steroid-sparing therapy (57, 58), other anti-IL-5/anti-IL-5 receptor and anti-CD52 antibody approaches for the treatment of HES remain investigational (8, 59, 60). The gene discussed is CD52; the disease is hypereosinophilic syndrome.