Ideally the CLuc assay solution is prepared immediately before performing the assays. However, the unused portion of the assay solution can be stored at -20°C for up to 2 days. It must be completely thawed to room temperature (protected from light) and mixed well (do not vortex) before use. Using the previously frozen assay solution will result in approximately a 20% decrease in CLuc activity. The unused portion of the CLuc assay solution should be discarded after one round of freeze-and-thaw.