Some of stem cell researcher Hwang Woo-suk's high-profile human cloning work announced earlier this year may have been "fabricated," a former top collaborator charged as he attempted to distance himself from the groundbreaking research.

University of Pittsburgh researcher Gerald Schatten has demanded that the journal Science remove him as the senior author of a report it published in June to international acclaim that detailed how individual stem cell colonies were created for 11 patients through cloning.

"My careful re-evaluations of published figures and tables, along with new problematic information, now casts substantial doubts about the paper's accuracy," Schatten wrote in a letter to Science released late Tuesday by the university. "Over the weekend, I received allegations from someone involved with the experiments that certain elements of the report may be fabricated."

Hwang could not immediately be reached for comment. An e-mail sent to his office at the start of the business day in Seoul was not returned.

Pitt spokeswoman Jane Duffield said Schatten wouldn't make any further comment while the university investigated the matter.

Schatten's highly unusual demand, in a letter that Science confirmed receiving Tuesday, adds to growing skepticism over Hwang's findings and places the entire cloning and stem cell field under a cloud.

"It's a very serious step. It's not good," said Rudolf Jaenisch, a leading stem cell scientist at the Whitehead Institute for Biomedical Research in Cambridge, Mass.

Many stem cell scientists had stood by Hwang's work even as the South Korean admitted to ethical lapses and minor data reporting errors.

Now, a significant number, such as Jaenisch, are calling on Hwang to submit his cloning research to independent analysts to bolster public confidence, which they perceive as eroding because of the continued controversy over Hwang's work.

Schatten set off the ethics furor last month when he publicly accused Hwang of collecting eggs from subordinate scientists, a practice many consider unethical, and lying about it to him.

But until now, even Schatten has maintained that the main findings of the paper — that tailor-made stem cells were extracted from embryos cloned from the DNA of sick volunteers — were valid.

The journal Science acknowledged receiving Schatten's demand, but spokeswoman Ginger Pinholster declined to release Schatten's letter because "it contains unsubstantiated allegations."

Schatten's name was listed last among the 25 authors, signifying that he was the senior researcher on the project.

"No single author, having declared at the time of submission his full and complete confidence in the contents of the paper, can retract his name unilaterally, after publication," the journal said in a statement.

The journal has said it has no reason to believe Hwang's primary finding "is any way fraudulent or questionable."

Stem cell scientists hope to clone embryos to extract stem cells in order to better learn how diseases develop and even perhaps rejuvenate failing organs. The basic idea of cloning is to take a patient's genetic material and inject it into an unfertilized human egg. The implanted DNA then drives the egg to develop into an embryo.

Hwang is considered a national hero in South Korea for his cloning prowess. He has publicly apologized for the ethical lapses and quit as head of the World Stem Cell Hub, an international project he had launched in October that envisioned California and British labs in addition to a facility in Korea.

At the time, Hwang defended his work as scientifically sound.

But doubts about his central claim — that he extracted separate stem cell colonies from 11 patients from cloned human embryos — are growing.

Colleagues in South Korea are also demanding independent confirmation of Hwang's results, and Hwang has been in and out of the Seoul National University hospital since last week, suffering from "extreme stress," hospital officials said.

The critics allege that Hwang's assertion, published by Science, that he created 11 separate cloned embryos may be inaccurate. That's because each cloned embryo was required to have a separate DNA sample showing it was derived from a unique individual. Several of the samples appear to be identical — suggesting more than one came from a single person.

If the critics are correct, the DNA data could simply be another careless reporting error akin to a photography mix up the Hwang team disclosed to Science last week. Or, the alleged DNA inaccuracy could be the tip of something more sinister.

Hwang's bosses at Seoul National University — prodded by 30 skeptical faculty members — have launched an investigation. Separately, the University of Pittsburgh has begun its own probe.

Stem cell scientists say an investigation from an outside party is necessary and could be done quickly.

Ian Wilmut, the man who cloned Dolly the sheep in 1998, and seven other scientists on Tuesday called on Hwang to submit his research to an independent analysis — essentially a simple paternity test that would prove all 11 stem cell lines were derived from separate patients.

Wilmut said in a letter to Science that he was able to fend off baseless fraud accusations over Dolly's nativity by handing over his data to an outside expert.

"It's an easy test that can be done in a few hours," said Dr. Bob Lanza, a cloning expert at the biotechnology company Advanced Cell Technology in Worcester, Mass.

Lanza co-wrote the letter Wilmut sent to Science.

"You can't fake the results if they're carried out by an independent group," Lanza said. "I think this simple test could put the charges to rest."