ERBB2 and breast cancer: Although ADCs have not currently been approved for the treatment of HER2-mutated metastatic BC, based on the previously reported clinical activity of ADCs alone and ADCs in combination with irreversible TKIs in HER2-mutated lung cancer and the findings of ongoing trials evaluating ADC efficacy in metastatic BC with HER2 mutations, we speculated that our patient would benefit from the addition of T-DM1 to her regimen.