Trastuzumab (HerceptinTM) is a monoclonal IgG1 antibody that targets the HER2/neu receptor, is known to be able to internalise and has been used successfully in the treatment of HER2-positive breast cancer.13 It is also the antibody component of trastuzumab emtansine (KadcyclaTM), a recently FDA-approved ADC therapy for the same indication.14 We thus rationalised that the Fab fragment of trastuzumab, 11, would be an appealing antibody fragment system on which to evaluate novel drug-linker 10. Here, ERBB2 is linked to breast cancer.