Based on the diagnostic workup, namely the esophagogastroduodenoscopy, colonoscopy, spirometry, plasma DAO enzyme activity analysis, and ruling out other differential diagnoses, like Celiac disease, lactose intolerance, and inflammatory bowel diseases (Crohn’s disease and ulcerative colitis), the patient was diagnosed with eosinophilic esophagitis and histamine intolerance. The gene discussed is DAO; the disease is ulcerative colitis.