PercevalHR is a genetically encoded fluorescent sensor of the ATP:ADP ratio, and can be used to image energy status in live cells. It is tuned to the range of intracellular ATP:ADP expected in mammalian cells. PercevalHR contains three tandem copies of the ATP-binding protein encoded by the Methanococcus jannaschii glnK1 gene; circularly permuted monomeric Venus has been inserted into the T loop (between residues Tyr51 and Ile52) of the first copy of glnK1, and the second and third copies each lack the T loop sequence. In addition, each copy of the glnK1 sequence contains amino acid substitutions compared to wild-type (PMID:24096541). For detailed kinetic and spectral properties, see PMID:24096541.