The c-erbB-2 receptor tyrosine kinase proto-oncogene product is overexpressed in 20-30% of breast carcinomas and this has been shown to correlate with poor prognosis. Previous analysis of tumour-derived lines has demonstrated that although the c-erbB-2 gene is often amplified, overexpression can occur from a single-copy gene. Moreover, whether or not the gene is amplified, overexpressing cells produce 6- to 8-fold more mRNA per gene copy than low-expressing cells. In this paper, we examine the possible mechanisms causing this deregulation of c-erbB-2 mRNA accumulation. Nuclear run-on studies indicated that the extra mRNA accumulation was due to increased transcription of the gene in overexpressing cells. Promoter analyses using c-erbB-2 5' flanking sequences linked to CAT showed that the promoter is more active in overexpressing cells. Coupling promoter deletion functional studies with footprinting experiments, using nuclear extracts derived from both low and overexpressing cells, allowed the identification of a DNA-binding protein, OB2-1, which is considerably more abundant in a range of overexpressing lines. We discuss the possible role of OB2-1 in c-erbB-2 overexpression in breast tumour lines.