text: Bioluminescence is the process of light emission in living organisms. Bioluminescence imaging utilizes native light emission from one of several organisms which bioluminesce, also known as luciferase enzymes. The three main sources are the North American firefly, the sea pansy (and related marine organisms), and bacteria like Photorhabdus luminescens and Vibrio fischeri.  The DNA encoding the luminescent protein is incorporated into the laboratory animal either via a viral vector or by creating a transgenic animal. Rodent models of cancer spread can be studied through bioluminescence imaging.for e.g.Mouse models of breast cancer metastasis.
  Systems derived from the three groups above differ in key ways:
  * Firefly luciferase requires D-luciferin to be injected into the subject prior to imaging. The peak emission wavelength is about 560 nm. Due to the attenuation of blue-green light in tissues, the red-shift (compared to the other systems) of this emission makes detection of firefly luciferase much more sensitive in vivo.
  * Renilla luciferase (from the Sea pansy) requires its substrate, coelenterazine, to be injected as well. As opposed to luciferin, coelenterazine has a lower bioavailability (likely due to MDR1 transporting it out of mammalian cells). Additionally, the peak emission wavelength is about 480 nm.
  * Bacterial luciferase has an advantage in that the lux  operon used to express it also encodes the enzymes required for substrate biosynthesis. Although originally believed to be functional only in prokaryotic organisms, where it is widely used for developing bioluminescent pathogens, it has been genetically engineered to work in mammalian expression systems as well. This luciferase reaction has a peak wavelength of about 490 nm.
  While the total amount of light emitted from bioluminescence is typically small and not detected by the human eye, an ultra-sensitive CCD camera can image bioluminescence from an external vantage point.