While basal eosinophilopoiesis under homeostatic conditions is controlled by the interplay of specific transcription factors including cytosine-cytosine-adenosine-adenosine-thymidine (CCAAT)/enhancer-binding protein (C/EBP), GATA binding protein 1 (GATA-1), PU.1, and friend of GATA (FOG) (reviewed in [33]), blood and tissue eosinophilia in eosinophilic diseases is regulated by the lineage-specific cytokine IL-5 which is produced by activated type 2 T helper cells (TH2) cells, mast cells, or type 2 innate lymphoid cells (ILC2s). The gene discussed is IL5; the disease is Increased total eosinophil count.