The evidence that PD-L1 is expressed on the surface of CTCs in breast cancer patients with ER or PR(+) and HER2(-) was first found by Mazel and his colleagues (26), and the established assays of CTC/PD-L1 will be used for liquid biopsy in future clinical experiments of breast cancer patients who are experiencing the immune checkpoint. This evidence concerns the gene ERBB2 and breast cancer.