ERBB2 and breast cancer: In agreement, our data from BC cell lines indicate that T-DM1 treatment induces internalization of membrane HER2 receptor in trastuzumab-resistant and pertuzumab-resistant BC cells lines; on the contrary, the expression levels of HER2 are not affected by T-DM1 in trastuzumab+pertuzumab-resistant BC cells, since the amount of membrane HER2 is already reduced by the previous double-block determining HER2 nuclear localization.