SMAD4 and metastatic neoplasm: Differentiating primary MOC from metastatic tumors originating from the pancreaticobiliary tract is a complex process, primarily due to the overlapping marker positivity rates between these two tumor types.PAX8, Deleted in Pancreatic Carcinoma 4 (DPC4), also known as Smad4, and Cytokeratin 17 (CK17) serve as critical differential markers.