Through a mutational analysis of a differentially regulated enhancer, we present evidence that supports a role for the transcription factor YY1 in tumor suppression in HeLa/fibroblast somatic cell hybrids. The human ST5 gene was previously shown to be expressed as three RNA species, 4.6, 3.1 and 2.8 kb in length. Whereas the two larger species are expressed at similar levels in all cell lines examined, the 2.8 kb mRNA is expressed specifically in non-tumorigenic hybrids. In this study, the basis for the differential expression of this mRNA species was investigated. The message was shown to originate from a promoter located within an intron of the ST5 gene. An enhancer located approximaely 1500 nt upstream of the start site was required for cell type specific expression. Mutational analysis of this enhancer revealed an AP1 site and five YY1 sites which were necessary for full enhancer activity. Levels of YY1 DNA binding activity were found to be as much as 6-fold higher in the non-tumorigenic cells relative to the tumorigenic cells, while AP1 activity was similar in both cell types. These results suggest that a signaling pathway targeting YY1 may play an important role in tumor suppression in HeLa-fibroblast hybrids.