CXCR4 and neoplasm: Consequently, the expression of FAS surface markers, such as the Fibroblast Activation Protein (FAP), Platelet-Derived Growth Factor Receptor (PDGFR), and C-X-C Chemokine Receptor Type 4 (CXCR4), can vary widely, reflecting the unique interactions between tumor cells and the surrounding stroma in each individual case [100].