Chalfie and colleagues showed that the green fluorescent protein (GFP) from the jellyfish Aequorea victoria, could be used as a visible marker for protein localization and expression in vivo, in bacteria and worm cells, and in the absence of any contributing factors from the jelly fish itself, proving that the protein acted totally alone without any interference from possible contaminating factors. This demonstration established GFP as a fine tool to study proteins in vivo, and fundamentally altered the way in which investigators could define and study intracellular and intra organismal biological events. Douglas Prasher played a key role in this discovery. Order of authorship in the original paper: Chalfie, Tu, Euskirchen, Ward, Prasher.

In 2008 Chalfie shared the Nobel Prize in Chemistry with Osamu Shimomura and Roger Y. Tsien "for the discovery and development of the green fluorescent protein, GFP." 

(Thanks to Juan Weiss for this reference and its interpretation.)