KRT18 and pancreatic ductal adenocarcinoma: To further investigate this phenomenon, we used IHC staining to measure the levels of expression of CK18 (cytokeratin 18, a marker of epithelial cell differentiation [40]), Ki-67 (a marker of cell proliferation [41]), and NeuroD1 in highly differentiated and poorly differentiated pancreatic ductal adenocarcinomas.