Bioluminescence Cell Spectral Imaging. HT1080 cells expressing various BRET fusion proteins and RLuc8 or RLuc8.6 were plated in 96-well plates in increasing variable numbers (200–60,000). After 24 h, the luciferase substrate (2 μg/well), CLZ (Nanolight Technology), or CLZ-v (Promega) was added. Cells were imaged immediately after the addition of substrate using an IVIS-200 or IVIS-Spectrum imaging system equipped with a cooled charge-coupled device camera (Caliper). Sequential imaging was performed using open/donor/acceptor/open filters. Spectral imaging was performed from 460 to 720 nm in 20-nm increments (each spectral filter used spans for 20-nm wavelength and is denoted by the midpoint); when EnduRen (Promega) was used as substrate, it was added 4 h before imaging. Regions of interest were drawn over each well for all filter sets, and the average radiance was determined. The equations used for calculating BRET ratios and DRs are provided in SI Materials and Methods.