By using the xenograft model in which human breast tumor cells were introduced to T cell-deficient mice, we demonstrated that the tumor-intrinsic activities of PD-L1 are cardinal for the ability of the cancer cells to form primary tumors and metastasize, and that they depend on the integrity of the S283 residue located in the intracellular domain of human PD-L1. The gene discussed is CD274; the disease is cancer.