ESR1 and breast cancer: IHC and western blot analysis confirmed the expression of Brca1 and ERα in regenerated mammary tumors by p18−/− tumor cells and lack of Brca1 and ERα in regenerated tumors by p18−/−;Brca1MGKO cells with E2 supplement (Additional file 3: Figure S3 and Additional file 4: Figure S4B, C).