PDCD1 and breast cancer: The development of immunotherapy for patients with breast cancer has been limited by the low immunogenicity of tumor antigens; however, recent studies combining chemotherapy with anti-checkpoint antibodies such as programmed cell death protein 1 (PD-1) and cytotoxic T lymphocyte-associated protein 4 (CTLA-4) (1–4) show that this combination induces better treatment efficacy against solid tumors, including breast cancer.