Overexpression of the ERBB2 oncogene is observed in about 30% of breast cancers and is generally correlated with a poor prognosis. Previous results from our and other laboratories indicated that elevated transcriptional activity contributes significantly to the overexpression of ERBB2 mRNA in mammary adenocarcinoma cell lines. Activator protein 2 (AP-2) transcription factors account for this overexpression through two recognition sequences located 215 and 500 bp upstream from the transcription start site. Furthermore, AP-2 transcription factors are highly expressed in cancer cell lines overexpressing ERBB2. In this report, we examined the cooperative effect of Yin Yang 1 (YY1) on AP-2-induced activation of ERBB2 promoter activity. We detected high levels of YY1 transcription factor in mammary cancer cell lines. Notably, we showed that YY1 enhances AP-2alpha transcriptional activation of the ERBB2 promoter through an AP-2 site both in HepG2 and in HCT116 cells, whereas a carboxyl-terminal-truncated form of YY1 cannot. Moreover, we demonstrated the interaction between endogenous AP-2 and YY1 factors in the BT-474 mammary adenocarcinoma cell line. In addition, inhibition of endogenous YY1 protein by an antisense decreased the transcription of an AP-2-responsive ERBB2 reporter plasmid in BT-474 breast cancer cells. Finally, we detected in vivo AP-2 and YY1 occupancy of the ERBB2 proximal promoter in chromatin immunoprecipitation assays. Our data thus provide evidence that YY1 cooperates with AP-2 to stimulate ERBB2 promoter activity through the AP-2 binding sites.