Differential expression can be used to identify genes that are likely to be involved in the development and progression of cancer. In order to detect genes whose expression is decreased in prostate cancer, we combined two methods: suppression subtractive hybridization and complementary DNA library arrays. Screening of the subtracted cDNA library using array hybridization resulted in eight different clones that we confirmed to be truly differentially expressed. Seven of them represented known genes, and one of them was an anonymous expressed sequence tag (clone 1B10), matching the chromosomal region 7q21. In northern blot analysis, the expressed sequence tag 1B10 hybridized to a 7.5-kilobase transcript. The expression of 1B10 seems to be quite prostate-specific: by northern blot analysis it is expressed, in addition to prostate tissue, only in ovary tissue. By quantitative polymerase chain reaction with reverse transcription, the expression of 1B10 is also detected in other tissues, but at tenfold lower levels than in prostate. The only prostate cancer cell line that expresses 1B10 is an androgen-sensitive cell line, LNCaP, suggesting that the gene might be androgen-regulated. Cloning of the full-length cDNA has so far resulted in 4.6 kb of the transcript. The 3′ end of the coding region is significantly homologous to a prostate-specific transmembrane protein, STEAP, but the rest of the sequence does not show homology to any known gene. Cloning of the 5′ end of the cDNA and functional studies are now in progress.