Förster resonance energy transfer (FRET)-based Cl indicators consist of two fluorescent proteins, Cyan fluorescent protein (CFP) and YFP connected via a polypeptide linker. This allows ratiometric Cl measurements based on the Cl sensitivity of YFP and Cl insensivity of CFP. Clomeleon and Cl Sensor are FRET-based Cl indicators that allow ratiometric non-invasive monitoring of chloride activity in living cells.