Karl Illmensee claimed he cloned mice — and the data was fabricated

In 1981, Karl Illmensee, a developmental biologist at the University of Geneva, published a paper in Cell claiming to have cloned mice by transferring nuclei from embryonic cells into enucleated eggs. If true, it would have been the first demonstration of mammalian cloning — a landmark achievement.

But other researchers could not replicate the results. An investigation by the University of Geneva and the journal Cell found that the data had been fabricated. The paper was retracted in 1984. Illmensee's co-author, Peter Hoppe, had raised concerns about the data before publication but was overruled.

The fraud set back the field of mammalian cloning by over a decade. It was not until 1996 that Dolly the sheep was successfully cloned by Ian Wilmut's team at the Roslin Institute. The case illustrated how a single fabricated paper can delay an entire field, and how the pressure to be first can drive a researcher to fabricate results that the scientific community desperately wants to be true.