Woo Suk Hwang has admitted to fabricating key parts of a study that purported to show the creation of the first human master cells tailor-made to match individual patients, according to Sung Il Roh, a senior colleague at his laboratory in Seoul, South Korea. Dr Sung said that 9 of the 11 colonies of stem cells featured in the study, which was published to worldwide acclaim in May in the journal Science, had not been authentic. The validity of the other two is uncertain.

He said that Professor Hwang had confessed to flaws in the study when Dr Sung visited him yesterday in hospital, where the scientist is being treated for exhaustion. Both researchers agreed to ask Science to retract their paper.

“Professor Hwang admitted to fabrication,” Dr Sung told MBC, a Korean television station. “Hwang said there were no cloned embryonic stem cells at all and he did not know that.” The allegations, to which Professor Hwang has yet to respond, cast fresh doubt on one of the most significant scientific breakthroughs of the year.

Professor Hwang, who created the first cloned human embryo in 2004, had claimed to have gone further and generated cloned ES cells that carried the DNA of people suffering from conditions such as Parkinson’s disease and diabetes. Such cells could potentially be transplanted into these patients to replace diseased tissue without risk of rejection. This process, sometimes known as “therapeutic cloning”, is one of the most exciting frontiers in medicine.

Concerns about the work, however, began to surface last month, when Gerald Schatten, of the University of Pittsburgh, ended his association with the Korean laboratory after it emerged that it had used eggs donated by junior researchers. This practice is widely thought to be unethical because of the potential for coercion, and the scandal led Professor Hwang to resign from an international cloning consortium.

Professor Schatten did not at first dispute the validity of the original research, but he changed his mind this week after re-examining the data. On Tuesday he said that he had become aware of serious flaws and that he had asked Science to remove his name from the paper while urging Professor Hwang to retract it.

“My careful re-evaluations of published figures and tables, along with new problematic information, now casts substantial doubts on the paper’s accuracy,” Professor Schatten wrote in a letter to the journal.

A Science official said last night that it had not received any request for a retraction from Korea, although it had contacted Professor Hwang to seek clarification. She said: “Science editors have asked Dr Hwang and his co-authors for clarification regarding unconfirmed news reports about requests for retraction.”

Dr Sung, who provided Professor Hwang with the eggs he used for his cloning experiments, said that while the scientist appeared to have created cloned stem cells in the first place, most of these had died because of a viral infection. Instead of admitting this, he said, the team substituted other stem cell lines and manipulated data to make it look as if they shared the DNA of the patients from whom they were supposed to have been cloned. The remaining two lines of stem cells were frozen, but it was not known whether they had survived.

“He told me that there were no stem cells,” Dr Sung said after visting Dr Hwang yesterday. “Without any cloned stem cells, he presented stem cells taken from my hospital. Nine of the eleven stem cell lines he had said he created didn’t even exist.”

Several cloning experts, including Ian Wilmut, who led the team that created Dolly the sheep, yesterday urged Professor Hwang to submit his work to independent scrutiny.

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