The c-erbB-2 oncogene is frequently overexpressed in human breast cancer partly due to its elevated transcription level. The promoter regions of the c-erbB-2 gene could therefore activate the transcription of a linked foreign gene preferentially in breast cancer cells. Previous reports showed that the 533-bp (-495/+38, +1 corresponds to the transcription start site) or the 251-bp (-213/+38) genomic fragment included the cis-acting elements which stimulated the transcription of a fused gene in breast cancer cells. Our previous study also indicated that the 251-bp fragment could transcribe the reporter gene better than the 533-bp fragment and that the 124-bp (-86/+38) region did not support the transcription of a linked reporter gene. In this study, we precisely analyzed the promoter activity of the genomic region between -213/+38 and -86/+38 in breast cancer, non-breast cancer cells and fibroblasts, and found that deletion of 22-bp from the 251-bp fragment markedly decreased the transcriptional activation in breast cancer cells. Although the 22-bp deletion also decreased the promoter activity in non-breast cancer cells, the deletion did not influence the activity in fibroblasts. Since the promoter activity of shorter genomic fragments beyond the 22-bp deletion remained low in breast cancer cells, the -213/-191 region contains a cis-acting element(s) that is minimally required for the preferential expression in breast cancer cells.