To study Her2.BBz and Her2.BBz.PD-L1 CAR T cell exhaustion after constant antigen exposure, we used the experimental scheme illustrated in Figure 4A. After prolonged exposure to HCC1954-Luc breast cancer cells, the proliferation ability of Her2.BBz CAR T cells was significantly decreased compared with that of Her2.BBz.PDL1 CAR T cells (Figure 4B). Here, ERBB2 is linked to breast cancer.