MUC5AC and mucinous neoplasm: CK20 is predominantly found in the lower gastrointestinal tract (e.g., colon and rectum) and associated malignancies (17), while MUC-5ac, a member of the mucin family, is typically upregulated in gastric mucosa and certain pancreatic mucinous tumors, making a negative result useful for ruling out typical mucinous tumor components or gastric-type differentiation (18).