Thus, we observed that copper‐induced activation of NF‐κB played a major role in boosting PD‐L1 expression in vitro and in vivo, accompanied by increased CD8+ cells in mammary tumors derived from MMTV‐PyMT mice, which was notably reversed by the treatment of copper chelator TTM or depletion of CTR1 (Figure 1E,F; Figure S3B–F, Supporting Information). This evidence concerns the gene SLC31A1 and breast cancer.