A set of contiguous and partially overlapping cDNA sequences and polypeptides encoded thereby, designated as CS198 and transcribed from GI tract tissue, is described. These sequences are useful for detecting, diagnosing, staging, monitoring, prognosticating, preventing or treating, or determining the predisposition of an individual to diseases and conditions of the GI tract, such as GI tract cancer. Also provided are antibodies which specifically bind to CS198-encoded polypeptide or protein, and agonists or inhibitors which prevent action of the tissue-specific CS198 polypeptide, which molecules are useful for the therapeutic treatment of GI tract diseases, tumors or metastases.