The invention features a method for inhibiting growth of a cancer cell by contacting the cell with a composition of an siRNA that inhibits expression of REG4 or KIAA0101. Methods of treating cancer are also within the invention. The invention also features products, including nucleic acid sequences and vectors as well as to compositions comprising them, useful in the provided methods. The invention also provides a method for inhibiting of tumor cell, for example pancreatic cancer cell, prostatic cancer cell, breast cancer cell, and bladder cancer cell, particularly pancreatic ductal adenocarcinoma (PDAC) by inhibiting REG4 gene. The present invention also relates to methods of treating or preventing PDAC in a subject comprising the step of administering to said subject a pharmaceutically effective amount of an antibody or fragment thereof that binds to a protein encoded by REG4. The present invention also relates to methods of diagnosing chemo-radiation therapeutic resistance of a cancer. The present invention also provides therapeutic agents or methods for treating cancer using the polypeptides. The polypeptides of the present invention are composed of an amino acid sequence which comprises polypeptide which comprises QKGIGEFF/SEQ ID NO: 21. The polypeptides of the present invention can be introduced into cancer cells by modifying the polypeptides with transfection agents such as poly-arginine.