ERBB2 and breast cancer: Cell proliferation assays showed that HER2-overexpressing cellsexpressing the AKT3 R247C mutant exhibit enhanced mTORC1 activity and increased trastuzumab tolerance, suggesting that AKT3 R247C may be involved in acquiring resistance to anti-HER2 therapy.[22] Tamoxifen-resistant human breast cancer cell lines show an increase in the expression and activity of AKT3.[157]