PGR and breast cancer: Additionally, Ser294 phosphorylation-induced antagonism of PR SUMOylation derepressed (activated) PR transcriptional activity at selected breast cancer-associated gene promoters, namely HBEGF [13], STC1 and IRS1 [16]; phospho-PR-dependent upregulation of the breast cancer-associated drivers, STC1 and IRS, occurred in the absence of progestins [16].