ERBB2 and breast cancer: Among the four molecular subtypes of breast cancer, ER‐positive/HER2‐positive breast cancer has the highest PIK3CA mutation rate (51.6%), followed by ER‐positive/HER2‐negative breast cancer (48.7%), and ER‐negative/HER2‐negative breast cancer has the lowest mutation rate (30.0%) but the highest PIK3CA amplification rate (19.05%) [15].