ERBB2 and breast cancer: To investigate whether these four miRNAs contribute to trastuzumab sensitivity of HER2+ breast cancer cells, we transfected HER2+ breast cancer cells lines (SKBR3 and BT474) with corresponding miRNA mimics or miRNA anti-sense oligonucleotides (ASOs) using Lipofectamine 3000 and subsequently treated the cells with trastuzumab at 10 μg ml−1 for 3 days.