PDCD1 and breast cancer: Immunotherapy is revolutionizing the therapeutic approach for solid malignancies, and accumulating data indicate that immune checkpoint antagonists such as programmed cell death-1 (PD-1)/programmed death ligand-1 (PD-L1) inhibitors can induce efficacious and durable clinical responses in a proportion of patients with breast cancer, especially metastatic breast cancer (Emens, 2018; Franzoi et al., 2021).