ERBB2 and breast cancer: To induce adaptation of HER2‐positive breast cancer to trastuzumab, we exposed trastuzumab‐sensitive HER2‐positive breast cancer cells (SKBR3) to gradually increasing concentrations of trastuzumab over a 30‐week period, starting from 1 μg ml−1 and reaching a final concentration of 20 μg ml−1 (Figure1a).