FAP(L5-MG.L91S.E52D) 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). FAP(L5-MG.L91S.E52D) binds as a homodimer to malachite green (MG) and induces it to exhibit far-red fluorescence. FAP(L5-MG.L91S.E52D) consists of a single VL immunoglobulin domain that carries two point mutations relative to the progenitor L5 VL domain (PMID:23978698, PMID:19606431).