ERBB2 and breast carcinoma: In this respect, it has been shown that the EGFR-dependent autocrine pathway plays a key role both in intrinsic or de novo resistance to tamoxifen in ER positive, HER2 overexpressing MCF-7 breast cancer cells and in the acquired resistance to tamoxifen in tamoxifen-treated MCF-7 cells (Nicholson et al, 2002; Knowlden et al, 2003; Shou et al, 2004).