Trastuzumab is used to treat HER2-positive breast cancer, but it is known to provoke cardiotoxicity because some HER2 is expressed, albeit at lower levels, in cardiomyocytes.28,29 We fused anti-idiotypic antibody fragments, which had been previously developed as vaccines for breast cancer by mimicking HER2/NEU,30,31 to prevent binding to HER2, as well as an antibody fragment that was not specific to trastuzumab, to probe the effect of steric hindrance alone. Here, ERBB2 is linked to breast cancer.