ERBB2 and breast cancer: Advances in the understanding of BC biology in recent decades and the deciphering of the plasticity of the clonal architecture of BC under the natural selection pressure applied by anticancer therapy [1–3] have led to the development and approval of several targeted agents, such as anti-HER2 (human epidermal growth factor receptor 2) therapy [4], mTOR- (mechanistic target of rapamycin) and CDK4/6 (cyclin-dependent kinase 4/6) inhibitors [5], as well as immunotherapy through checkpoint inhibition [6].