Studies by comparative genomic hybridization imply that amplification of the chromosomal region 17q22-q24 is common in breast cancer. Here,amplification and expression levels of six known genes located at 17q23 were examined in breast cancer cell lines. Four of them(RAD51C, S6K, PAT1, and TBX2) were found to be highly amplified and overexpressed. To investigate the involvement of these genes in vivo, fluorescence in situ hybridization analysis of a tissue microarray containing 372 primary breast cancers was used. S6K, PAT1, and TBX2 were coamplified in about 10% of tumors, whereas RAD51C amplification was seen in only 3% of tumors. Expression analysis in 12 primary tumors showed that RAD51C and S6K were consistently expressed in all cases in which they were amplified and also in some tumors without amplification. These data suggest that 17q23 amplification results in simultaneous up-regulation of several genes,whose increased biological activity may jointly contribute to the more aggressive clinical course observed in patients with 17q23-amplified tumors.