Regardless of whether you are working with soluble or membrane proteins, numerous crystallization conditions need to be screened and optimized to find the appropriate conditions for crystallization. One of the main factors for successful crystallization in Lipidic Cubic Phase (LCP) is the ability of the protein to diffuse within the lipid bilayer. The diffusion rate of the protein is affected by protein aggregation, structural properties of the LCP, and the chemical environment. The diffusion rate can be determined by Fluorescence Recovery After Photobleaching (FRAP), which measures the amount of time required for the fluorescence intensity of a tagged protein to reestablish itself within a small area in the LCP drop that has been subject to optical bleaching (Figure 1). The curve of normalized fluorescence intensity vs. time can be plotted and analyzed to quantify two diffusion-related characteristics: mobile fraction (the value that the intensity curve asymptotically approaches) and the diffusion rate (proportional to the slope at the starting point of the curve).