The ability of estrogen receptor alpha (ERalpha) to modulate gene expression is influenced by the recruitment of a host of co-regulatory proteins to target genes. To further understand how estrogen-responsive genes are regulated, we have isolated and identified proteins associated with ERalpha when it is bound to DNA containing the consensus estrogen response element (ERE). One of the proteins identified in this complex, proliferating cell nuclear antigen (PCNA), is required for DNA replication and repair. We show that PCNA interacts with ERalpha in the absence and in the presence of DNA, enhances the interaction of ERalpha with ERE-containing DNA, and associates with endogenous estrogen-responsive genes. Interestingly, rather than altering hormone responsiveness of endogenous, estrogen-responsive genes, PCNA increases the basal expression of these genes. Our studies suggest that in addition to serving as a platform for the recruitment of DNA replication and repair proteins, PCNA may serve as a platform for transcription factors involved in regulating gene expression.