Anti- Programmed cell death 1 (PD-1) or programmed death-ligand 1 (PD-L1) blocking antibodies have shown objective responses in a variety of solid tumors including melanoma, lung cancer, prostate cancer, breast cancer, ovarian cancer, head and neck cancer, and a subset of colorectal cancers (De Felice et al., 2015; El-Osta et al., 2016; Gong et al., 2016; Lynch and Murphy, 2016; Moskovitz et al., 2018; Calles et al., 2019). Here, PDCD1 is linked to breast cancer.