Calcium indicator, membrane impermeable. Compatible with flow cytometry, plate reader, and fluorescence microscopy assays. Excitation: 490 nm | Emission: 515 nm | Kd: 355 nM | Purity: >95% Fluo-4 is the most used and the most trusted of the single-wavelength Ca²⁺ indicators. It has been the indicator of choice for countless basic research studies and has been the used to screen many tens-of-millions of compounds using high-throughput screening (HTS) to discover pharmacological tools and drugs for GPCRs, and a wide variety of Ca²⁺ permeable ion channels and transporters. Fluo-4 K⁺ Salt is a membrane impermeable form of Fluo-4 that can be used in lipid membrane-free systems, in liposomes, or can be introduced into cells by electroporation, microinjection or other methods.