FAP and neoplasm: Thus, considering the high expression of FAP on the cell surfaces of activated CAFs and its limited expression in normal tissue, FAP-targeting ligands based on FAP inhibitors (FAPI) have recently been introduced [22,23], radiolabelled with either 68Ga or 18F, and several further FAPI variants were designed to increase tumor uptake and retention of these tracers in malignant cells [24].