Gain of chromosome arm 8q is a frequent genetic alteration in breast and prostate cancer. Two amplified subregions, 8q21 and 8q23-24, have been identified with comparative genomic hybridization (CGH). We have recently demonstrated that the EIF3S3 (eIF3-p40) gene, located at 8q23, is often amplified and overexpressed in both breast and prostate cancer. Here, we used fluorescence in situ hybridization (FISH) to map the amplified region around EIF3S3 in primary breast cancers and cell lines. The size of the common highly amplified region was about 2.5 Mb between the markers D8S1668 and WI-7959. Next, we analyzed the expression of all expressed sequence tags (ESTs) located within and near this region by RNA slot blot hybridization. In addition to EIF3S3, three anonymous ESTs and EXT1 were found to be highly expressed in cancer cell lines with the amplification at 8q23-q24. However, the anonymous ESTs were located outside the minimal highly amplified region and EXT1 was overexpressed only in one of the cancer cell lines with 8q amplification. Since EIF3S3 was the only consistently overexpressed gene located in the minimal highly amplified region, it is the strongest candidate target gene for 8q23-q24 amplification.