ChemiBrite cells express a novel variant of clytin, a calcium-activated photoprotein, to enable sensitive luminescent detection of ligand-induced calcium flux. The ChemiBrite version of clytin contains a mutation that increases its affinity for calcium to a level that permits detection of cytosolic calcium in many cells with greater sensitivity than other photoproteins targeted to the mitochondria. Luminescent calcium assays offer several advantages over fluorescent calcium assays including increased sensitivity and lack of interference from fluorescent compounds. Cloned HEK Parental Gαqo ChemiBrite cells were made by stable transfection of HEK293 cells with optimized quantities of plasmid encoding a novel variant of clytin and human Gαqo. These stability-tested cells are ideal for recombinant expression of target protein for use in calcium flux assays, for analysis of agonist, antagonist and modulator activity at the target protein, as well as cAMP assays.