To determine if the constellations of mutations would be distinct between the HER2-positive and HER2-negative components of HER2 heterogeneous breast cancers, and to identify potential driver mutations restricted to the HER2-negative components, we subjected the HER2-positive and HER2-negative components of three cases (that is, T6, T11 and T12), for which sufficient DNA from frozen tumor and matched normal tissues was available, to whole exome sequencing. This evidence concerns the gene ERBB2 and breast cancer.