Among the five cases with emerged HER2 mutations in metastatic lesions after one year of trastuzumab adjuvant treatment, two cases were identified with the K753E mutation, and the other three had the L755S mutation. K753E and L755S were confirmed to be drug-resistant mutations (resistant to trastuzumab and lapatinib). The mutation rate of HER2 was much higher than that in the primary tumor (2.24%, 28/1248).