Disclosed herein is a novel retinoblastoma binding protein-related gene that encodes an antigenic epitope, IKPSLGSKK (SEQ ID NO:3), which is differentially expressed in cancers, such as breast, lung, colon, pancreas, and ovarian cancer. Polynucleotides that encode and polypeptides that comprise the antigenic epitope and variants thereof are disclosed. Also, disclosed are methods of using the polynucleotides and polypeptides of the present invention in immunotherapies and immunoassays.