GRAB(5-HT1.0) is a genetically encoded fluorescent sensor for serotonin (5-HT) that can be used for measurement of endogenous serotonin dynamics in vivo. It is a G-protein-coupled receptor activation-based ('GRAB') sensor based on the serotonin receptor encoded by the Homo sapiens HTR2C gene (HGNC:5295); a circularly permuted form of GFP (cpGFP) has been inserted into the third intracellular loop, and a number of additional mutations have also been introduced during optimization of the sensor. Serotonin-dependent conformational changes in the receptor result in a change in fluorescence of the conformationally sensitive cpGFP; GRAB(5-HT1.0) shows an increase in fluorescence in response to extracellular serotonin (PMID:33821000).