ESR1 and cancer: E2/ERα-SMYD3, SHCBP1, H3K4me3 and Kras-MAPK in all developmental stages of mammary tissues in the Brca1MKO mouse and parous mammary epithelial cells and serve as a cancer driver in general, although a stronger effect is observed in early pregnancy because of the higher levels of E2/ERα signaling.