Immunohistochemical assay for MUC5AC (Mucin 5AC) aims to detect this specific type of mucin glycoprotein produced primarily in the surface epithelial cells of the stomach, cervix, and bronchi. It is used as a diagnostic marker for mucinous carcinoma of the stomach and pancreas, as well as mucinous ovarian cancer, and to help differentiate the origins of mucinous metastases.