Why didn't the scientists who created Dolly go to greater lengths to prove that she really is what they say she is, a lamb produced by cloning the udder cells of an adult sheep?

On Wednesday, PPL Therapeutics, the Scottish company that helped create Dolly, the cloned sheep, said it had cloned five pigs that lack one copy of the gene.

It had taken Dr. Wilmut 277 attempts to create Dolly, and there was a consensus that a similar effort to create people would be unworkable, and certainly unethical.

In the late 1990's, Dr. Yang mastered the same technique that the Scottish scientist Ian Wilmut had used to create Dolly, the cloned sheep born in 1996.

In 1966 he created his first picture books, Black Dolly and Sean and the Carthorse, both about mistreated working horses.

It took 277 attempts to create Dolly.

He said he expected to use cells from a live animal rather than those from the dead ewe that he used to create Dolly.

In creating Dolly, his goal - like that of many other researchers around the world - was to turn animals into factories churning out proteins that can be used as drugs.

"They've leaped over the Roslin stuff," said his colleague from the University of Saskatchewan, Reuben J. Mapletoft, referring to the lab in Scotland that created Dolly.

Wilmut, who led the team that created Dolly, announced in 2007 that the nuclear transfer technique may never be sufficiently efficient for use in humans.