ERBB2 and neoplasm: 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).