A researcher embroiled in a fabrication scandal that has rocked Japan's scientific establishment said Friday she would resign after failing to reproduce results of what was once billed as a ground-breaking study on stem cells.
Haruko Obokata said she was dismayed that new laboratory tests have not been able to repeat her experiments, which she had claimed showed the successful conversion of an adult cell into a stem cell-like state.
"I am keenly aware of my responsibility for troubling a number of people," Obokata said in a statement. "I even can't find the words for an apology."
Her resignation came as Japan's Riken Institute formally announced that so-called "STAP" cells cannot be recreated, apparently drawing a line under the controversial study.
"We have conducted verification experiments but can't repeat the STAP phenomenon," team leader Shinichi Aizawa told a news conference.
"As a result, we will terminate the verification experiments," he said.
Riken in January trumpeted Obokata's simple method to re-programme adult cells to work like stem cells -- precursors that are capable of developing into any other cell in the human body.
Her work was published in the international journal Nature.
The study was top news in Japan, where the photogenic and Harvard-trained Obokata became a phenomenon.
But media attention soon grew into scepticism as doubts emerged about her papers on Stimulus-Triggered Acquisition of Pluripotency (STAP).
Mistakes were discovered in some data published in two papers, photograph captions were found to be misleading, and the work itself could not be repeated by other scientists, leading to accusations the data had been doctored.
Obokata, who asserted that she created STAP cells some 200 times, has been trying since July, in tandem with independent teams, to reproduce her own results.
She claimed there was a secret technique for creating STAP cells, but has refused to publicise it, asserting it is a subject of her future papers.
"I worked hard for three months to show significant results... but I'm so exhausted now and extremely puzzled," Obokata said in the statement.
The Britain-based Nature withdrew the flawed study after Obokata agreed in June to retract the papers.
As the scandal deepened, Obokata's mentor and co-author, stem cell scientist Yoshiki Sasai, hanged himself, further shaking Japan's scientific establishment.
Riken has pledged to restructure its Center for Developmental Biology where the scandal took place.