We next performed targeted massively parallel sequencing, using a panel of 273 genes frequently mutated in breast cancer and DNA repair-related genes [48], of the HER2-negative and HER2-positive components of 5 HER2 heterogeneous breast cancers (that is, T1, T3, T4, T8, and T9; Figure 5C). This evidence concerns the gene ERBB2 and breast cancer.