Stem Cell Paper

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TOKYO—The co-author of two breakthrough papers on making stem cells said Monday he had asked for the papers to be retracted, citing "crucial mistakes" in the research.

The decision is the latest and perhaps heaviest blow against the research, which was published in Nature in January and has been under intense scrutiny in recent weeks.

A spokeswoman for Nature said the journal was still investigating the matter and declined further comment.

Some scientists have said they have had trouble replicating the results, and Japan's Riken Center for Developmental Biology, where the work took place, opened an investigation last month into alleged irregularities in images used in the papers.

Teruhiko Wakayama of Yamanashi University in Japan, a co-author of both papers published in Nature, said he has asked the lead author of the papers, Haruko Obokata, to retract them.

"There is no more credibility when there are such crucial mistakes," he said in an email to The Wall Street Journal. He didn't elaborate on what the mistakes were.

Dr. Wakayama said the stem cells he had received from Dr. Obokata may not have been what he thought they were. The authors said they had converted mouse blood cells into an embryonic-like state simply by dipping them in a mild acid solution, creating what they called STAP cells, for stimulus-triggered acquisition of pluripotency.

Dr. Wakayama has said he was in charge of creating chimera mice that had cells from both the host embryo and the STAP cells. On Monday, he said he wasn't sure he actually received STAP cells and said he was handing over the cells in his possession to a third party for investigation. "I myself don't know what I used in my experiments," he said in the email.

Riken confirmed that Dr. Wakayama had asked for a retraction…