GRAB(rACh1h) is a genetically encoded red fluorescent sensor for acetylcholine (high affinity). It is a G-protein-coupled receptor activation-based ('GRAB') sensor derived from the acetylcholine receptor encoded by the Mus musculus Chrm3 gene (MGI:MGI:88398); an N-terminal portion of the gene has been replaced with the corresponding region (residues 2-198) from the Homo sapiens CHRM3 gene (HGNC:1952) in an effort to improve the response and affinity of the sensor to acetylcholine, and a circularly permuted form of mApple (cpmApple) has been inserted into the third intracellular loop (ICL3) of the Chrm3 gene. A number of additional mutations have been introduced into both the acetylcholine receptor and fluorescent protein sequences to optimize the sensor. Acetylcholine-dependent conformational changes in the receptor result in a change in fluorescence of the conformationally sensitive cpmApple (https://www.biorxiv.org/content/10.1101/2024.12.22.627112v1).