Green Fluorescent Protein
A valuable tool of the cell biologist and/or mad scientist. Can make mice glow green, but is usually found in its natural state in the jellyfish Aqueorea victoria. The structure was determined (in 1997) to be a beta-can, a particularly stable form of up-down beta barrel. The flurophore is formed from three cyclised amino acids in the shielded environment of the can's interior.
Also, blue fluorescent protein and other GM variants.