ERBB2 and breast cancer: For instance tumors of patients with a higher risk for disease progression and/or breast cancer‐related death have: (a) high expression of HER2 but low or intermediate number of ER+ cells (P2 in Fig. 3), (b) a mixture of cells with different HER2 CN levels (G3 in Fig. 4), and (c) a mixture of cells with different HER2 CN levels with low number of ER+ cells (PG3 in Fig. S5).