Many molecular factors related to the carcinogenesis and malignant behavior of pancreatic cancer were identified by investigation of intraductal papillary mucinous tumors (IPMTs). Molecules controlling the G1/S phase, such as p53, p21, p61, and cyclin D1, tumor suppressors including DPC-4, and many other factors such as MUC-1/2 contribute to the promotion of the malignant behavior of IPMTs. They will be important molecular targets for the radical treatment of pancreatic cancer.