Stockholm - Three US-based researchers shared the Nobel prize for chemistry Wednesday for work on developing a key tool used for tagging bioscience processes, the Royal Swedish Academy of Sciences said in Stockholm.

The three - Osamu Shimomura of Japan and US citizens Martin Chalfie and Roger Y Tsien - were cited for "the discovery and development of the green fluorescent protein, GFP."

The protein is a key tool in bioscience and used to "watch processes that previously were invisible," the academy said, noting for instance the development of nerve cells in the brain or cancer cells.

The glowing fluorescent protein was first observed in a species of jellyfish, Aequorea victoria, in 1962.

The three share the prize worth 10 million kronor (1.5 million dollars).

Shimomura - born 1928 - a retired professor with the Marine Biological Laboratory (MBL), Woods Hole, Maryland in the US was cited for isolating GFP from the jellyfish that drift with the currents off the west coast of North America.

He discovered that this protein glowed bright green under ultraviolet light.

Chalfie - born 1947 - is with Columbia University of New York and "demonstrated the value of GFP as a luminous genetic tag for various biological phenomena," the academy said.

Tsien - born 1952 - is professor at the University of of California, San Diego, La Jolla, "extended the colour palette beyond green allowing researchers to give various proteins and cells different colours," the academy said.

The different colours allow researchers to track different biological processes at the same time.

Tsien told reporters at the academy headquarters via a loudspeaker telephone, that he was "very happy about the recognition of our work. I did not really expect it. There were rumours of course, but from sources that were somehow quite questionable."

Tsien said he "was not a Chinese scientist. I grew up in the US and spent my life here ever since. But if it makes people in China, I hope the prize will inspire students and scientists there."

On Tuesday, US researcher Yoichiro Nambu and his Japanese colleagues Makoto Kobayashi and Toshihide Maskawa won the physics prize for describing the smallest building blocks in nature and nature's order within the framework of spontaneous broken symmetry.

On Monday, the medicine prize was awarded to German Harald zur Hausen for discovering the human papilloma virus which causes cervical cancer, while French scientists Francoise Barre-Sinoussi and Luc Montagnier, were awarded for their discovery of the human immunodeficiency virus (HIV) that causes acquired immune deficiency syndrome (AIDS).

The winners of the literature prize is to be named on Thursday and the peace prize on Friday.

The prizes were endowed by Swedish industrialist Alfred Nobel, the inventor of dynamite.

The economic sciences prize - a prize not endowed by Nobel and awarded since 1968 - is slated to be announced on Monday.

The award ceremonies are held December 10, the anniversary of Nobel's 1896 death in San Remo, Italy.