The ST7 gene was cloned and mapped to chromosome 7q31.1-q31.2, a region suspected of containing a tumor suppressor gene involved in a variety of human cancers. Subsequent investigation described the presence of ST7 mutations in human cell lines derived from breast tumors and primary colon carcinoma. Introduction of the ST7 cDNA into a prostate cancer-derived cell line abrogated in vivo tumorigenecity in nude mice. To clarify the role of the ST7 gene in cancer, we scrutinized primary head and neck squamous cell carcinomas, invasive ductal carcinomas of the breast, and adenocarcinomas of the colon. Loss of heterozygosity of D7S522/D7S677 was detected in 24% (4 of 17) of head and neck squamous cell carcinomas, 17% (2 of 12) of invasive ductal carcinomas of the breast, and 33% (8 of 24) of adenocarcinomas of the colon, but no somatic mutations were found in any of these specimens. We then searched for mutations in breast cancer cell lines and found a complete wild-type sequence in all, including cell lines previously reported to harbor mutations. We believe that the ST7 gene is not a primary target of inactivation in most human cancers with loss of heterozygosity at 7q31.1-q31.2.