The ability of physiological doses of metformin to enhance the antitumoral functionality of T-cells, neutralize immune-inhibitory cell populations residing in the tumor microenvironment, downregulate the immune-checkpoint PD-L1, and influence gut microbiota composition [140] might provide a/the right path to reconsider the incorporation of metformin into the breast oncologist’s armamentarium, hand in hand with cancer immunotherapy. This evidence concerns the gene CD274 and cancer.