Fig. 1

FCS monitors the random motion of fluorescently labeled molecules. These fluctuations provide information on the diffusion time of a particle and are directly dependent on the molecular size. For a large molecule, the fluorescence fluctuation is slow (a), whereas a small molecule makes rapid fluctuations (b). Consequently, any increase in the mass of a molecule (for example, as the result of an interaction with a second molecule) is readily detected as an increase in the particle’s diffusion time. An actual G(τ) plot curve of Rho6G is shown as an example (c)