FAPα2 is an engineered antibody that acts as a fluorogen activating protein (FAP); a FAP binds specifically and with high affinity to a small-molecule 'fluorogen' to form a complex that shows strong fluorescence, with the spectral properties being determined by the particular combination of FAP and fluorogen. Neither the FAP nor the fluorogen are fluorescent by themselves (PMID:18157118, reviewed in PMID:31789501, PMID:29881673).