Though evidence points to the existence of STAP cells, they are still only a hypothesis worthy of study, a co-author of the papers on the revolutionary but unproved method to create pluripotent stem cells said Wednesday.

In his first public appearance since questions were raised about the alleged discovery of STAP (stimulus-triggered acquisition of pluripotency) cells, Riken researcher Yoshiki Sasai said work must be done to verify the "hypothesis" now that the credibility of the papers has been lost as a result of errors and defects.

"To retract the papers would be the most appropriate step to take for now due to credibility issues," Sasai said at a press conference in Tokyo.