A controversial cloning scientist is to announce he has created "human-cow" embryos that lived for about a fortnight and theoretically could have been implanted into a woman's womb.
Panayiotis Zavos, who runs a fertility laboratory in the US, made the hybrid embryos by inserting human DNA into the eggs of a cow. Professor Zavos said the human-cow embryos were "theoretically viable" but emphasised that he had no plans to allow such a hybrid to be born.
"We are not trying to create monsters," he said, claiming his aim was to perfect his cloning techniques without the ethical problems involved in the use of human egg cells.
Professor Zavos said the embryos grew to several hundred cells and appeared to have normal DNA. He will announce the breakthrough at a meeting in London today.
He added that the hybrids had grown beyond the stage, known as differentiation, at which cells showed the first signs of developing into tissues and organs.
This is a crucial test because it implies that the human chromosomes, which contain the DNA, have been left intact. Other people who claim to have created human clones have failed to produce evidence. Professor Zavos's work is taken seriously by some of the scientific community.