Because of these precedents and the fact that HER3 is frequently expressed in breast cancer (Vollmann‐Zwerenz et al, 2010), we decided to explore the potential therapeutic value of the HER3 targeting ADC EV20/MMAF in HER2+ breast cancer, with particular focus on the action of that drug on cells resistant to trastuzumab and other anti‐HER2 therapies. This evidence concerns the gene ERBB3 and breast cancer.