Figure 1. A live cell nucleus containing hormone-induced green fluorescent protein–tagged glucocorticoid receptors (GFP-GR) and 200 tandemly repeated copies of a promoter array for GR (a). The array appears as a bright spot marked here by the yellow circle. In fluorescence recovery after photobleaching (FRAP) experiments, fluorescence is specifically bleached only inside the yellow circle, and then the rate at which fluorescence recovers there is monitored (b). Since the rate of fluorescence recovery is rapid, narrow strip images (red rectangle in part a above) are acquired on a confocal microscope, thereby reducing the scan time for acquisition of each image during the fluorescence recovery. Images at a few selected recovery time points are shown (b). Using all the collected time points, the average intensity inside the yellow circle is measured to generate a FRAP curve (c). The curve is normalized to one based on the initial intensity inside the yellow circle. Curves such as this can be fit to estimate values for the in vivo binding parameters of GR at a promoter.