CELT-211 is a potent, selective fluorescent ligand for the h5HT2B serotonin receptor, demonstrating full selectivity over h5HT2A and h5HT2C receptors with a Ki value of 56.3 nM in radioligand binding assays for h5HT2B receptors. Its excitation and emission maxima are 589 nm and 616 nm, respectively, making its wavelengths compatible with its application as an acceptor dye in Time-Resolved Fluorescence Resonance Energy Transfer (TR-FRET) assays when paired with the CoraFluor 1 TR-FRET donor.