Considering the lower levels of HER2 expression in HER2 1 + or HER2 2 + and ISH-negative tumors and the possibility of targeting these tumors with novel anti-HER2 drug antibody conjugates [11, 12], we witnessed the emergence of a new subgroup of breast tumors called “HER2-low breast cancer” [13, 14]. This evidence concerns the gene ERBB2 and breast cancer.