Fluorescence correlation spectroscopy measures molecular transport in cells

Fluorescence correlation spectroscopy (FCS) can measure dynamics of fluores cent molecules in cells. FCS measures the fluctuations in the number of flu orescent molecules in a small volume illuminated by a thin beam of excitati on light. These fluctuations are processed statistically to yield an autoco rrelation function from which rates of diffusion, convection, chemical reac tion, and other processes can be extracted. The advantages of this approach include the ability to measure the mobility of a very small number of mole cules, even down to the single molecule level, over a wide range of rates i n very small regions of a cell. In addition to rates of diffusion and conve ction, FCS also provides unique information about the local concentration, states of aggregation and molecular interaction using fluctuation amplitude and cross-correlation methods. Recent advances in technology have rendered these once difficult measurements accessible to routine use in cell biolog y and biochemistry. This review provides a summary of the FCS method and de scribes current areas in which the FCS approach is being extended beyond it s original scope.