The possible explanations might include: low-level HER2 expression in ER-positive patients can possibly induce treatment resistance due to the crosstalk between HER2 and ER pathway, and the role of HER2 low expression may interact with ER-positive breast cancer but not in ER-negative patients, as estrogen has little effect in the ER-negative group. This evidence concerns the gene ERBB2 and breast cancer.