To further investigate if crosstalk between ER and HER2 is a mechanism of resistance to HER2-targeted therapy in vivo, using UACC-812 xenografts we compared the efficacy of the anti-HER2 regimens alone (Figure 6A) to block tumor growth versus their efficacy in combination with estrogen deprivation (ED) to also inhibit the ER pathway (Figure 6B). This evidence concerns the gene ESR1 and neoplasm.