FAP and cancer: Targets included fibroblast activation protein (FAP), a cell surface-bound collagenase and cancer biomarker20, which removes the dipeptide AP from the N terminus of polypeptides; TEV protease and enterokinase, both well-studied endopeptidases that are absent in the central nervous system and could, therefore, be used as bio-orthogonal markers for transgenesis or cell tracking21, 22; and caspase-3 (CASP3), another endopeptidease and cytosolic apoptosis actuator that can be released following apoptosis and secondary necrosis23 and serve as a prognostic indicator during cancer treatments24, 25.