Three scientists at American universities have won the 2008 Nobel Prize in Chemistry for discovering a fluorescent protein in a colorful jellyfish and developing it into a key tool for observing previously invisible processes such as the spread of cancer cells, the Royal Swedish Academy of Sciences announced this morning.

The winners are Osamu Shimomura, of the Marine Biological Laboratory, in Woods Hole, Mass., and the Boston University Medical School; Martin Chalfie, of Columbia University; and Roger Y. Tsien, of the University of California at San Diego. The three men will share the prize, worth about $1.4-million this year, equally.

The award reflects a trend in recent years in which the chemistry prize has recognized advances on the biological frontiers of the chemical sciences.

Mr. Shimomura is being honored for being the first scientist to isolate the protein, known as green fluorescent protein, or GFP, from the Pacific jellyfish Aequorea victoria in 1962 and discovering that GFP glows bright green under ultraviolet light.

Mr. Chalfie showed how GFP could be used to tag a range of biological objects, from cells to other proteins, in order to observe processes and other phenomena that would otherwise go unseen. In one of his first experiments, according to the Nobel citatation, he used GFP to mark six cells in the roundworm Caenorhabditis elegans, the common lab animal.

Mr. Tsien helped explain why GFP is fluorescent and figured out how to make GFP show up in colors other than green, allowing different cells or proteins to be tagged with different colors in the same experiment and thereby permitting different processes to be tracked simultaneously.

The three winners will collect their prizes at a ceremony in December. —Andrew Mytelka