bioluminescence is not susceptible to environmental factors. We applied CAL as an energy‐donor substrate for a bioluminescence resonanceenergytransfer (BRET) system with click beetle red luciferase (CBRluc), a mutant of firefly luciferase, as the energy‐donor enzyme and yellow fluorescent protein (YFP) as the energy‐acceptor fluorophore, and obtained a clearly bimodal bioluminescence spectrum. Stable