Firefly luciferin \Fire"fly` lu*cif"er*in\, n. (Biochem.) a type of {luciferin} produced by the firefly {Photinus pyralis}. Note: Its structure has been elucidated, and chemically it is recognized as 4,5-Dihydro-2-(6-hydroxy-2-benzothiazol-2-yl)-4-thiazolecarboxylic acid, {C11H8N3O2S2}. It has found use in a very sensitive assay for ATP, in which concentrations of ATP as low as 10^{-11} molar can be detected. --[MI11] [PJC]