ESR1 and breast cancer: Given identification of progesterone receptor (PR) binding sites within this super enhancer, PR’s previously identified role in regulating KRT5 expression in luminal breast cancer cells48,60, and finally its overexpression in multiple ESR1 mutant cell models25–27,61 (Supplementary Fig. 10d, e), we tested whether PR regulates KRT14/16/17 expression.