To identify novel, tissue-restricted cell surface proteins in cancer which can serve as targets for antibody-based diagnostics and therapeutics, a translated version of the expressed sequence tag database (tblastn) was mined for transcripts with similarity to the glycoprotein A33 (GPA33) colon cancer antigen. A novel human transcript, termed A34, was identified which encoded a putative cell surface protein, GPA34, which is approximately 30% identical to GPA33 and other members of the junctional adhesion molecule (JAM) family. Conventional end-point and quantitative real-time RT-PCR showed that A34 mRNA expression is highly tissue-restricted, as it is expressed predominantly in stomach and testis. A34 mRNA was also detected in 6/19 (31%) gastric cancers, 8/16 (50%) esophageal carcinomas, and 4/17 (23%) ovarian cancers, but not in lung, breast or colon carcinomas. A murine monoclonal antibody (mAb A34) was generated to the extracellular domain of the A34 protein and used to biochemically and immunohistochemically characterize the A34 antigenic system. The mAb A34 specifically recognized glycoproteins ranging in apparent size from 55-70 kDa, present in normal gastric mucosa and in COS-7 cells transfected with A34 cDNA. Of 31 different normal tissues examined by immunohistochemistry, GPA34 protein expression was detected primarily in normal stomach mucosa and testicular germ cells, and in the tumor cells of 5/17 (29%) gastric cancers, 7/11 (63%) esophageal cancers, and 2/21 (9%) ovarian cancers, in agreement with gene expression results. The A34 antigen and monoclonal antibody may be of considerable value for immunotherapy of different types of cancer.