D-Luciferin is a chemiluminescent substrate of firefly luciferase. It produces light upon oxidative decarboxylation by luciferase in the presence of ATP. D-luciferin can be employed 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.