ERBB2 and breast cancer: In this study, we found that serum soluble PD-L2 (sPD-L2) concentration was elevated in breast cancer patients compared with healthy controls by enzyme-linked immunosorbent assay (ELISA), and the expression of PD-L2 was positively correlated with progesterone receptor (PR) and human epidermal growth factor receptor 2 (HER2) by immunohistochemistry experiments in breast cancer patients.