PDCD1 and breast cancer: The synergistic effect of anti-PD-1 agent on trastuzumab therapy has been demonstrated in a HER2-positive BC mouse model (59), and another study found that the combination of ado-trastuzumab emtansine (T-DM1) and anti-cytotoxic T-lymphocyte-associated protein 4 (CTLA4) or anti-PD-1 antibodies elicited responses in HER2+ BC xenografts that had previously been resistant to T-DM1 monotherapy (60).