Figure 1 FCS plus allows assay miniaturization to 1-µl formats.

To demonstrate FCS+plus multiparameter read-outs, a model system based on biotinstreptavidin was used. Binding of fluorescently labeled biotin (molecular weight 1 kD) to streptavidin (molecular weight 60 kD) can easily be monitored by FCS. This allows one to determine the diffusion rates of free and bound biotin, and the distribution of fluorescent biotin between these two species. A test run was carried out in a miniaturized

format in an HTS mode (l-µl sample volume, 2-sec read time per well) using unlabeled biotin as a competitor.