GRAB(ACh1.0) is a genetically encoded fluorescent sensor for acetylcholine that can be used for real-time detection of endogenous acetylcholine dynamics. It is a G-protein-coupled receptor activation-based ('GRAB') sensor derived from the acetylcholine receptor encoded by the Homo sapiens CHRM3 gene (HGNC:1952); the third intracellular loop (ICL3) of the CHRM3 gene has been replaced with the shorter ICL3 sequence from the Homo sapiens ADRB2 gene (HGNC:286) (to avoid any potential problems with expression and trafficking of the protein) and a circularly permuted form of EGFP (cpEGFP) has been inserted into this ICL3 loop. Acetylcholine-dependent conformational changes in the receptor result in a change in fluorescence of the conformationally sensitive cpEGFP; GRAB(ACh1.0) shows an increase in fluorescence in response to extracellular acetylcholine (FBrf0240517).