CD274 and neoplasm: TAM agonist homologous ligand protein S (PROS1) secreted by TAMs can bind to TAM receptors MER proto-oncogene, tyrosine kinase (MERTK), AXL and TYRO3, which promote malignant characteristics of tumor cells and promote immune escape by upregulating PD-L1 expression (Burstyn-Cohen, Maimon and Signaling 2019).