Abstract: Nucleic acid compositions and polypeptides encoding a red-shifted form of firefly luciferase are provided. These red-shifted luciferases are characterized by spectrum of light emission having detectable emissions at 610 nm (luc610), preferably a primary peak at 610 nm. The nucleic acid compositions find use in various systems as a reporter gene, and are of particular interest for use as a reporter with in vivo systems, because of the efficient transfer of red light through tissues. The red-shifted luciferase may be combined in such assays with luciferases emitting at other spectra, in order to monitor multiple processes simultaneously.