To identify possible driver events in the HER2-negative components of HER2 heterogeneous breast cancers, of which >90% were ER-positive and of luminal A-like or luminal B-like subtype according to the St Gallen International Expert Consensus 2013 [31] (Additional file 1), the copy number status and gene expression of the 1,535 genes found to be amplified only in the HER2-negative components of the cases analyzed here were assessed in the luminal A and luminal B breast cancers of the TCGA dataset [10]. This evidence concerns the gene ESR1 and breast cancer.