PHILADELPHIA (AP) -- After stunning the world by creating the first adult clone, scientist Ian Wilmut hinted he may be repeating the experiment to silence critics who question whether Dolly the sheep actually came from an adult cell.
Critics were moving too quickly to dismiss his claim, said Wilmut, of the Roslin Institute in Scotland.
"There hasn't been time for other labs to complete their work and get it published," he said Friday while speaking at the American Association for the Advancement of Science.
Asked why he hasn't duplicated the Dolly experiment, Wilmut replied, "Perhaps we are." He did not elaborate and would not respond to further questions.
Wilmut set off a worldwide debate about the ethics of cloning last year by announcing that he had cloned Dolly using the nucleus from the cell of an adult sheep. It marked the first time that a genetically identical mammal had been created from an adult cell, and spurred argument about cloning of humans.
President Clinton issued orders that no federal dollars could be spent to attempt to clone humans. Several bills that would ban or put a moratorium on human cloning have been introduced in Congress, and at least 20 states also are considering such laws.
But last month, two researchers suggested in a letter to the journal Science that Wilmut's claim about Dolly could be false.
Dr. Norton Zinder of Rockefeller University and Dr. Vittorio Sgaramella of the University of Calabria in Italy questioned the credibility of the Wilmut's claim because at least three labs have failed in attempts to duplicate the work. They also questioned why the Scottish lab has not repeated the experiment. Scientific experiments usually must be repeated before they are verified and accepted by other scientists.
The Dolly claim also has been challenged because the frozen specimen came from a pregnant sheep and it is possible that a fetal cell in the sheep's blood could have supplied the genes that created the clone. Fetal clones have been performed by a number of labs.
Wilmut said it is "extraordinarily unlikely" that Dolly came from a fetal cell because such cells in the maternal blood are rare -- only one among several million adult cells.
The Scottish scientist said he remains opposed to attempting to clone humans.
"I have not heard a reason for doing that that I find ethically acceptable," he said.
Another speaker at the conference, Lori Andrews of the Chicago College of Law, said some of the state legislation seeking to forbid human cloning would leave loopholes permitting it.
For instance, experiments have shown that eggs from cows could be used "as universal incubators" for human clones, and at least eight proposed state laws would not forbid this practice.
She said it also is possible to clone using a very early human embryo, which would be permitted under loopholes in six proposed state laws.
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