Compared with luminal breast cancer, the HER2 subgroup exhibited a significantly higher percentage of M and E + M CTCs per 4 mL of blood (mean, HER2: 60.80% vs. luminal A: 24.66%, P < 0.05; HER2: 60.80% vs. luminal B: 26.20%, P < 0.05) (Figure 4F). This evidence concerns the gene ERBB2 and breast cancer.