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After sequestering himself in a hospital psychiatric ward earlier this week, embattled South Korean cloning pioneer Woo Suk Hwang finally held a press conference on Friday. Before a packed room at Seoul National University, Hwang defiantly denied multiple allegations that he had fabricated the results of his history-making stem cell study published earlier this year in the journal, Science. During his lengthy statement, he blamed his compromised results on the possibility that his stem cell lines were switched at some point. He didn't elaborate on how or why the switch occurred.

But less than an hour later after Hwang's statement, his primary critic and former colleague, Dr. Roh SuNg-Il, held his own press conference. He accused Hwang of lying and told reporters: "A liar does not remember his previous lies."

While Hwang defends his credibility, he admitted to making "a lot of mistakes" and being "negligent in maintaining the stem cells." He also asked Science to withdraw his paper because of these lapses may have compromised his data. Editors at Science confirmed that they have received requests from both Hwang and a co-author, Dr. Gerald Schatten of the University of Pittsburgh, to withdraw the paper. The journal requires all 25 co-authors to agree to a retraction, and Hwang said the remaining requests were forthcoming.

Despite the retraction, Hwang insists he will provide proof of his work in 10 days. Hwang maintains that he did create the groundbreaking stem cell lines from patients, and that he even provided them at the time to six researchers who can vouch for their authenticity. "What I can say for sure is that we did create patient-specific stem cells and we have source technologies to repeat them from scratch," he said, referring to the stem cells he cultured for the first time from 11 patients with diseases.

But Hwang did not specifically address the most serious charge which aired on the Korean network MBC-TV on Thursday night. In an interview, one of his junior researchers claimed that Hwang had asked him to turn two or three stem cell lines into 11 by photographing them repeatedly for submission to Science.

Instead, Hwang acknowledged that the images published in Science were problematic, but provided his own explanation: that the stem cells were somehow mismanaged or possibly switched. In January, his team at Seoul National University had created six patient-specific stem cell lines, but they became contaminated. They reported the breach to the government and rescued two batches which they stored temporarily at their partner lab in Mizmedi Hospital. After Hwang's lab was cleaned and new sterilization procedures put into place, the stem cell lines came back to Hwang's lab. His team then went on to create six more lines in another cycle attempt, and then three additional lines in a subsequent cycle. Hwang's practice was to grow the delicate cells in their initial stages in his lab, then move them to Mizmedi for storage and further maturation.

When producers of an investigative television program from MBC-TV raised questions about the validity of the stem cells following the egg-donor scandal, Hwang provided MBC-TV with samples from five stem cell lines, and cells from their donors, in an effort to prove their authenticity. It's not clear where these stem cells were stored. Hwang said he and his colleagues performed their own in-house test, including DNA fingerprinting to verify the source of the stem cells. It was then that he discovered that the fingerprints did not match those printed in Science. "We learned that the stem cells were actually made in Mizmedi," he said, and therefore were not, in fact, the stem cells Hwang had created from the patients.

His team is thawing five frozen stem cell lines and conducting DNA analysis to verify that he was indeed successful in using cloning techniques to extract stem cells from patients with diseases; those results will be available in 10 days. After that, Hwang will have either answered his critics or he'll have even more questions to face.

• Alice Park
• South Korean stem cell pioneer insists his groundbreaking research is still credible