Moreover, the A/D measured in mice is independent of the reporter cell number (total bioluminescence signal) for both BRET6.1 (Fig. 3E) and RLuc8.6 (Fig. 3F) as long as the level of donor signal exceeds the background signal associated with spontaneous chemiluminescence of CLZ by at least twofold. The BRET6 system seems to reach the detection limit for