GRAB(ACh4l) is a genetically encoded fluorescent sensor for acetylcholine (low affinity). It is a G-protein-coupled receptor activation-based ('GRAB') sensor based on the acetylcholine receptor encoded by the Homo sapiens CHRM3 gene (HGNC:1952) and containing a circularly permuted form of EGFP (cpEGFP) inserted into an intracellular loop.