Fluorescence from the beads was easily detectable by fluorescence photomicroscopy when the amount of translation reaction added to an immunoprecipitation corresponded to as little as 1 nl per bead (Fig. 1c). This is equivalent to about 2 × 10 6 protein molecules per bead, giving a surface density of about 80 molecules per ␮m 2. Even a density as low as about 8 molecules per ␮m 2 could be seen i...