PDCD1 and breast cancer: The FDA-approved CDK4/6 inhibitor abemaciclib has shown preclinical synergistic antitumor effects with PD-1 inhibitor in breast cancer mouse models, the ID8 murine ovarian cancer model, and the colon adenocarcinoma murine model, which depends on increased T-cell recruitment with elevated levels of TH1 cytokines/chemokines (198–200).