Science 324, 804–807 (2009)

A team led by Roger Tsien of the University of California, San Diego, reports that it has engineered the first protein that emits infrared light and can be used to image intact animals.

Existing fluorescent proteins are excited by shorter wavelengths, which don't penetrate far into animals' bodies. The new proteins were made from a light-detecting pigment called a phytochrome from the bacteria Deinococcus radiodurans. The phytochrome naturally incorporates a green pigment, biliverdin, that is abundant in animal tissues. Tsien's team modified the phytochrome so that it rigidifies biliverdin, which then absorbs far-red light and emits infrared light.

The researchers showed that the modified phytochrome can be used to image an animal's inner tissues, such as the liver, and say that it could be useful in fields such as cancer and stem-cell research.