This invention provides nucleic acids containing sequences from a TCR.gamma. transcript from prostate epithelial cells and many breast cancer cells and a T-cell receptor gamma Alternate Reading frame Protein ("TARP") expressed from the translation of those sequences. Vaccines made from TARP are useful in raising immune responses to cells in which the protein is expressed, including prostate cancer cells and cells of many breast cancers. The invention also provides methods for diagnosing the presence of prostate cancer and TARP-expressing breast cancers, as well as methods of administering TARP and nucleic acids encoding TARP to subjects.