Immunohistochemical MTAP loss consistently co-occurred with homozygous CDKN2A deletion and was present in 25.5% of cholangiocarcinomas, 9.3% of esophageal adenocarcinomas, 10.3% of gastric carcinomas, and 30.2% of pancreatic adenocarcinomas. The gene discussed is CDKN2A; the disease is cholangiocarcinoma.