MUC5AC and neoplasm: Notably, CLDN18.2 expression was more prevalent and frequent than mucin 5AC (MUC5AC), an early marker for pancreatic ductal tumors, and claudin-18.2 had a specificity and sensitivity of 93% and 79%, respectively, for identifying the stomach, pancreas, and biliary tract as the primary tumor site, with the positive and negative predictive values of 76% and 94%, respectively.