PDCD1 and breast cancer: Primary tumors from MMTV-PyMT mice, or resulting from E0771 breast cancer cell transplantation, did not respond to either programmed cell death protein 1 (PD-1) or cytotoxic T-lymphocyte antigen 4 (CTLA-4) inhibitors alone, or in combination with ALK1-Fc (Supplemental Figure 1, B and C).