Mucin 5AC (MUC5AC) is a secretory-type mucin found in columnar mucous cells of surface gastric epithelium and in goblet cells of the fetal and precancerous colon, but not in normal colon cells. MUC5AC expression is indicated in carcinomas wherein the type is defined as diffuse and infiltrative, and those located mainly in the antrum. Studies have also suggested a correlation between MUC5AC and colorectal signet-ring cell carcinoma, with overexpression of MUC5AC relating to the carcinogenesis, malignant potential, progression, and clinical behaviors.