ERBB2 and breast cancer: Studies of variations in the molecular subtypes of breast cancer have shown that the frequency of ER-positive breast cancer is the highest, and ranges from 40 to 50% in the luminal A and B subtypes [18], and in other subtypes it can vary significantly and ranges from 10 to 25% in HER2 positive, while it amounts to 13–40% in triple negative [23].