Moreover, to corroborate these results, we assessed P-gp levels using IHC in tumor samples derived from an orthotopic mouse model injected with D538G or Y537S ER cells (generated by us previously and described in detail [12]) and noted higher P-gp expression in D538G-ER tumor cells compared to Y537S-ER (Fig. 4e). This evidence concerns the gene ESR1 and neoplasm.