ERBB2 and breast carcinoma: Besides, both p-Akt and p-Erk were found to be overexpressed in ZR-75-1 as well as BT-474 cells after fulvestrant treatment but notably decreased from these levels when combined with trastuzumab, implying that the combination of trastuzumab and fulvestrant significantly prevented fulvestrant from increasing the levels of these proteins in HR+/HER2+ breast cancer.