GRAB(rACh0.5) is a genetically encoded red fluorescent sensor for acetylcholine. It is a G-protein-coupled receptor activation-based ('GRAB') sensor that is similar to GRAB(ACh3.0), except that a circularly permuted red fluorescent protein (cpmApple) is inserted into an intracellular loop of the Homo sapiens CHRM3 gene (HGNC:1952), rather than cpEGFP (PMID:39827209).