As co-localization of these receptors has previously been shown on PR binding sites in the promoter of the progestogen-responsive proto-oncogene MYC in response to MPA in the T47D breast cancer cell line (24), we first investigated the regulation of MYC mRNA expression in response to our panel of progestogens in the MCF-7 BUS cell line, and subsequently whether these ligands would induce PR and ERα co-localization on the MYC promoter. This evidence concerns the gene PGR and breast cancer.