Our findings for the first time reveal the bidirectional nature of the transcriptional interaction between AP-1 and Stat3 and their interacting cofactors, PR and ErbB-2, which function in the manner of an enhanceosome, that is, an array of transcription factors (AP-1 and Stat3), whose response elements are clustered in the DNA, plus their interacting cofactors (PR and ErbB-2) and co-activators (P300 and CBP) that function cooperatively, in this case to induce cyclin D1 promoter activation upon progestin stimulation of breast cancer cells. This evidence concerns the gene CCND1 and breast carcinoma.