Firefly luciferin (D-luciferin) is the luciferin, or light-emitting compound, found in many firefly (Lampyridae) species. D-luciferin is the substrate for firefly luciferase (Photinus pyralis). Luciferase catalyzes the oxidation of Firefly luciferin to oxyluciferin in the presence of ATP and magnesium, resulting in bioluminescence. This bioluminescent reaction is the most efficient known in nature, with about 90% of the energy released being converted to light. The luciferin/luciferase system is used as a very sensitive reporter assay for gene expression. It can also be used for ATP assays in research applications or to detect bacterial contamination. A luciferase-based ATP-TCA (ATP-tumour chemosensitivity test) to measure ATP levels was designed in the 1990s. This method permits the effects of different medicines and their combinations to be assessed in tumour biopsy specimens and has been used for chemotherapy drug selection for the treatment of ovarian cancer, cervical cancer, and breast cancer as well as melanoma and hepatoma.