Fluo-4 is used to measure calcium (Ca2+) concentrations inside living cells, and is often used for high-throughput screening of receptor ligands and calcium permeable ion channels. The green-fluorescent calcium indicator, Fluo-4, is an improved version of the calcium indicator, Fluo-3. It is commonly used as the non-fluorescent acetoxymethyl ester (Fluo-4 AM) which is cleaved inside the cell to give the free, fluorescent Fluo-4. It loads faster, is brighter at equivalent concentrations and is well-excited by the 488 nm line of the argon-ion laser which is often used in biological research laboratories. The sole distributor and owner of the patents is Invitrogen which acquired the original inventor Molecular Probes.