We determined expression of ERα by FACS and found that all cells derived from four individual p18−/−;Brca1MGKO and p16−/−;Brca1MGKO tumors were ERα negative whereas, as controls, 59% of MCF7 and 43% of p18−/− tumor cells were ERα positive, respectively (Additional file 2: Figure S2E). Here, ESR1 is linked to neoplasm.