ERBB2 and cancer: Clinically, all invasive BCs are grouped into following biomarker‐defined subtypes/groups for treatment purposes: (1) ER‐positive, HER2‐negative, (2) ER‐positive, HER2‐positive, (3) ER‐negative, HER2‐positive, and (4) ER‐negative, HER2‐negative cancers.20