StayBrite™ D-Luciferin sodium salt is a useful substrate for the enzyme Firefly Luciferase. Upon oxidation by the enzyme luciferase, produces bioluminescence. D-Luciferin can be utilized to assay the expression of the luciferase gene linked to a promoter of interest. Alternatively, D-Luciferin and luciferase can be used to assess ATP availability in cellular or biochemical assays.