Coelenterazine 400a is a derivative of native coelenterazine and serves as an excellent substrate for Renilla luciferase (Rluc). However, it cannot be oxidized by Gaussia luciferase (Gluc). Its maximum emission wavelength is around 400 nm, which minimizes signal interference with the GFP receptor protein. This makes it an important coelenterazine-based substrate for BRET (bioluminescence resonance energy transfer) studies.