A co-author of two controversial stem cell papers questioned the claims they made Monday, saying a new study showed that the cells purportedly used in the experiments did not appear to be STAP cells after all.

"There is no proof as to the existence of STAP cells," University of Yamanashi professor Teruhiko Wakayama told a news conference. "All the results of the analysis are moving toward denying the existence of STAP cells but I cannot say that the cells absolutely do not exist."

Wakayama added that he tried a dozen times to create STAP cells at his university but failed.