In healthy subjects, the upper normal limit of eosinophils in the peripheral blood is 3–5% (absolute eosinophil count of 350–500/mm3).[5,6] Extensive infiltration of eosinophils in tissue results in organ damage through associated tissue fibrosis via the secretion of TGF-β and IL-1β.[7,8] A chronic state of eosinophil proliferation and infiltration can lead to a diagnosis of HES. The gene discussed is IL1B; the disease is hypereosinophilic syndrome.