A systematic review reported that blood-based biomarkers such as peripheral blood AEC, eosinophilic cationic protein (ECP), eosinophil-derived neurotoxin (EDN), eosinophil peroxidase (EPX), and major basic protein (MBP) play a crucial role in the eosinophil activity and inflammation associated with EoE. The gene discussed is EPX; the disease is eosinophilic esophagitis.