CD274 and cancer: The expression of PD-L1 in cancer cells is controlled by several signaling pathways and proteins often mutated or up-regulated during malignant transformation, including COX2/mPGES1/PGE2 [56], hypoxia-inducible factor alpha (HIF1α) [57,58,59], nuclear factor NF-kappa-B p105 subunit (NF-κB) [60], PI3K/AKT/mTOR [61,62], RAF/MEK/ERK/MAPK [63,64,65,66] pathway, and STATs [67,68].