Since our data revealed that Smyd3 and ERα signaling is elevated in both MTP12 cells and breast cancer tissues, we hypothesized that the actions of cancerous factors or pathways that contribute to tumorigenesis and progression could be stimulated by elevated ERα and Smyd3 signaling in rapidly proliferating MTP12 mammary epithelial cells during early pregnancy in mice. This evidence concerns the gene ESR1 and breast cancer.