Amplification of human epidermal growth factor receptor 2 (HER2) occurs in 20% of breast cancers and confers an aggressive tumour phenotype with a poor prognosis. There is currently no cure for advanced HER2-positive breast cancer. By combining the targeting power of HER2-directed antibody (trastuzumab) with the cell killing capability of a novel cytotoxic molecule discovered in our group, it is possible to kill cancer cells more effectively. The antibody–drug conjugate compounds (ADCs) generated by this project ultimately have the potential to increase progression free survival rates in patients with HER2-expressing metastatic breast cancer. Even in the absence of being a ‘cure’, the technology has the potential to increase the quality of life for patients who are terminally ill, allowing them to have extra years of life where otherwise they may not.