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. Here, ERBB2 is linked to breast cancer.