iGluSnFR3(v857) is a genetically encoded fluorescent sensor designed for in vivo detection of glutamate release in neurological systems. It is a derivative of the SF-Venus-iGluSnFR-A184V sensor described in PMID:30377363, obtained by selection for improved specificity for synaptic versus extrasynaptic glutamate. iGluSnFR3(v857) is a fusion protein composed of an N-terminal IgG kappa secretion tag, a glutamate-binding region based on the Escherichia coli gltI gene, with a circularly permuted form of superfolder Venus inserted between residues 253 and 254, a Tag:MYC tag and a C-terminal transmembrane domain from PDGFR (which anchors the protein to the membrane and displays it on the extracellular side). iGluSnFR3(v857) contains 15 amino acid substitutions relative to SF-Venus-iGluSnFR-A184V (PMID:37142767). For a comparison of the iGluSnFR3(v857) sensor with other iGluSnFR variants, see PMID:37142767.