The validity of Hwang Woo-suk’s pioneering human cloning work in South Korea is in question as a former collaborator tries to distance himself from the groundbreaking research.

University of Pittsburgh researcher Gerald Schatten has demanded that the journal Science remove him as the senior author of the highly publicized report published in June that detailed how individual stem cell colonies were created for 11 patients through cloning.

Schatten’s highly unusual demand, in a letter that Science confirmed receiving Tuesday, adds significantly to the growing skepticism over Hwang’s findings and places the entire cloning and stem cell field under a cloud.

Unlike the earlier discussion about donated ova, this apparently goes to the merits of the research.