The present invention relates to a novel human gene that is differentially expressed in human carcinoma. More specifically, the present invention relates to a polynucleotide encoding a novel human polypeptide named C35 that is overexpressed in human breast and bladder carcinoma. This invention also relates to C35 polypeptides, as well as vectors, host cells, antibodies directed to C35 polypeptides, and the recombinant methods for producing the same. The present invention further relates to diagnostic methods for detecting carcinomas, including human breast carcinomas. The present invention further relates to the formulation and use of the C35 gene and polypeptides in immunogenic compositions or vaccines, to induce antibody or cell-mediated immunity against target cells, such as tumor cells, that express the C35 gene. The invention further relates to screening methods for identifying agonists and antagonists of C35 activity.