In addition, they show differences in immune biology and mutational load; for example, the HER2-positive (HER2+) BC and triple-negative breast cancer (TNBC) subtypes have a higher lymphocyte infiltration compared with ER-positive/HER2-negative (ER+/HER2−) tumours [3] and moreover, a higher mutational load is observed in ER-negative (ER–) tumours than ER+ tumours [4]. The gene discussed is ERBB2; the disease is breast cancer.