European Molecular Biology Laboratory,

embl-heidelberg.de Fluorescence spectroscopy approaches, previously confined to cuvette-based measurements, are progressively making their way into the field of cell biology. This novel development adds an aspect other than spatial resolution to microscopy – detection of protein activity in the cell. Mainly because of the availability of an ever-increasing range of intrinsically fluorescent proteins that can be genetically fused to virtually any protein of interest, the area of their application as fluorescent biosensors has reached the inner workings of the living cell. The most basic use of fluorescent biosensors is the collection of photons from a cell or tissue to detect the occurrence of a process with temporal resolution