The expression of the recently identified tumor suppressor gene, DBC2 (Deleted in Breast Cancer 2), is frequently extinguished in breast cancer cells or tissues. Mutation analysis of the essential promoter region, all exons and exon/intron boundaries of the DBC2 gene was performed in 100 sporadic breast cancer cases by PCR-SSCP, and DHPLC, followed by direct sequencing. An additional 17 breast cancer families, who were negative for the BRCA1/2 mutations, were analyzed by direct sequencing. Three novel mutations were observed in the promoter and 5'-untranslated region (UTR) of the gene; a germ-line G>A transition in the promoter at nt -238 from the transcription start site, and two tumor-specific mutations at nt -121C>T and nt +48G>A. No deleterious mutations were detected in the coding sequence of the gene in familial and sporadic breast cancer cases. The sequence variations found within the promoter and 5'-UTR region of the gene warrant expression analysis and screening more tumor samples at this region.