ERBB2 and breast cancer: Previous clinical trials which only enrolled primarily trastuzumab-resistant, HER2-positive advanced breast cancer added the mammalian target of rapamycin (mTOR) inhibitor [11–13], tyrosine kinase inhibitor (TKI) [14], phosphatidylinositol 3-kinase (PI3K) inhibitor [15–17], or programmed cell death-1 inhibitor [18] to anti-HER2 therapy with or without chemotherapy, but all showed disappointing clinical benefits.