The expression of LDH-A, an enzyme important in the production of lactic acid, and hypoxia-inducible factor-1α (HIF-1α), a major marker of hypoxia, was increased in breast cancer cells compared with normal breast cells; the expression of PD-L1 and pY-STAT3 was also increased in breast cancer cells (Fig. 5A). Here, STAT3 is linked to breast cancer.