Cancer/testis (CT) antigens are immunogenic in cancer patients, exhibit highly tissue-restricted expression, and are considered promising target molecules for cancer vaccines. To date, 44 CT gene families have been identified and their expression studied in numerous cancer types. For example, bladder cancer, non-small cell lung cancer, and melanoma are high CT gene expressors, with 11/20 (55%), 17/33 (51%) and 17/32 (53%) of the CT transcripts examined by RT-PCR detected in 20% or more of the specimens examined, respectively. Breast and prostate cancer can be considered moderate CT gene expressors, with 12/32 (37%) and 6/20 (30%) CT transcripts having an expression frequency >20%, respectively, while renal and colon cancer are low CT gene expressors, with only 3/33 (9%) and 4/25 (16%) CT transcripts having an expression frequency >20%, respectively. In normal tissues, standardized RT-PCR experiments showed that 19/43 CT genes were testis-restricted, 10/43 CT genes were tissue-restricted (mRNA detected in 2 or fewer non-gametogenic tissues), 9/43 CT genes were differentially expressed (mRNA detected in 3-6 non-gametogenic tissues), and 5/43 CT genes were ubiquitously expressed. With the exception of testis-restricted CT transcripts, all remaining CT transcripts were expressed in normal pancreas. In terms of immunogenicity, 14/29 testis/tissue-restricted CT gene families have been shown to induce a cellular and/or humoral immune response in humans. In view of the expanding list of CT genes, a CT gene database was created to standardize CT nomenclature and accumulate relevant data regarding their expression profiles, immunogenicity, function (where known), gene structure and location, and orthologous groups.