As only about one-third of breast cancer patients overexpressing HER2 respond to trastuzumab monotherapy, the identification of predictive biomarkers that can more accurately select responders or non-responders is vital, not only to improve its therapeutic index, but also to gain insight into the molecular pathways involved in trastuzumab resistance and to rationally design successful combination therapies. Here, ERBB2 is linked to breast cancer.