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Sperm protein 17 (Sp17) is a highly antigenic, testes-specific protein whose known function is to bind sperm to the zona pellucida. However, the Sp17 gene has been recently detected in normal non-testes tissues and malignant neoplasias. As the role of Sp17 in non-testes tissues is unknown, the characterization of the Sp17 gene in highly proliferating tissues may provide further insight into the regulation and alternative function of Sp17. Reverse transcriptase-polymerase chain reaction (RT-PCR) was used to amplify the Sp17-1 transcript in multiple normal human tissues and cancer cell lines. Similarly, the Sp17-2 gene was examined by PCR. In addition, Northern and Western blot analyses were used to detect Sp17 mRNA and protein expression. The Sp17-1a and Sp17-1b transcripts were amplified from cancer cell lines. Similarly, an Sp17-2 transcript was also detected in cancer cell lines. Furthermore, Northern blot analysis revealed Sp17 mRNA expression in all cancer cell lines examined. However, Sp17 protein expression was not detected. The differential detection of the Sp17 transcripts in cancer cell lines as compared to normal non-testes tissues, suggests a potential pathogenic role for Sp17 in diseased cells. Moreover, the Sp17-2 transcript may be a marker for highly proliferating cells. Collectively, these data implicate Sp17 as a cancer testis antigen. [Copyright &y& Elsevier]