In a letter to the journal Science, Norton Zinder of the Rockefeller University in New York and Vittorio Sgaramella of the University of Calabria in Italy said that since Dolly was presented to the public there had been "much agonising discussion, potential legislation and some laurels, but no more Dollies".
The criticism provoked a firm defence last night from Ian Wilmut, whose team at the Roslin Institute in Edinburgh produced Dolly. He said that it was unlikely anyone would reproduce the results in only 11 months because sheep have a five-month gestation period.
"It is unlikely that other authors would yet have had time to complete similar experiments and publish data," he said.
When Dolly the sheep was unveiled as the first cloned mammal, a ripple of excitement and consternation swept across the globe, resulting in fears overcloning humans.
In their letter, however, Zinder and Sgaramella said any results compiled from the cloning of Dolly from the cell of an adult ewe should not be accepted as dependable until the experiment was repeated.
That was standard procedure but no other scientists had reported cloning a mammal from an adult cell. The cloning was done just once in 400 attempts, they argued, adding: "Only one successful attempt out of some 400 is an anecdote, not a result. All kinds of imagined and unimagined experimental error can occur."
Dolly was cloned from DNA taken from a pregnant ewe, a process which Zinder and Sgaramella claim could have resulted in the critical DNA being extracted from a stray foetal cell.
As a result, the genetic evidence matching Dolly to the batch of donor cells was "good, but not sufficient". Zinder and Sgaramella also noted that Wilmut's report on Dolly had failed to say that the donor ewe was dead at the time of the experiment, preventing other testing to show Dolly was a clone.Reuse content