Proc. Natl Acad. Sci. USA doi:10.1073/pnas.0911116107 (2010)

Chemical labelling of molecules in living animals can provide insight into metabolic and disease processes, but attaching molecular tags rapidly without poisoning the subject can be tricky.

Carolyn Bertozzi at the University of California, Berkeley, and her colleagues have now shown that a method previously used to fluorescently label molecules in developing zebrafish also works in live mice.

They injected mice with a compound bearing a nitrogen-based chemical group called an azide, which bound to sialic acids found on cell surfaces. The animals were then injected with another compound, a cyclooctyne, which was linked to a fluorescent probe. The cyclooctyne reacted with the azide group, leaving a traceable label in the heart, liver and other tissues, seemingly with no toxic effects. Crucially, the authors' technique does not rely on the copper catalyst usually used in these sorts of reactions.