Robin Lovell-Badge, a genetics expert at Britain's National Institute for Medical Research, said the technology "to genetically manipulate pigs in this way would be very valuable." Lovell-Badge had not seen the research from China's cloned pigs and could not comment on its credibility. He said, however, that organs from genetically altered pigs would potentially solve some of the problems of rejected organs in transplant operations.

He said the presence of the green protein would allow genetically modified cells to be tracked if they were transplanted into a human. The fact that the pig's offspring also appeared to have the green genes would indicate that the genetic modification had successfully penetrated every cell, Lovell-Badge added.

But he said much more research and further trials - both in animals and in humans - would be necessary before the benefits of the technology could be seen.

Other genetically modified pigs have been created before, including by Scotland's Roslin Institute, but few results have been published.

Tokyo's Meiji University last year successfully cloned a transgenic pig that carries the genes for human diabetes, while South Korean scientists cloned cats that glow red when exposed to ultraviolet rays.

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Associated Press Writer Maria Cheng in London contributed to this report.