Four 'siblings' of Dolly the sheep have just passed their ninth birthday and are in good health, raising hopes that the cloning process does not detrimentally affect well-being as was feared following Dolly's early death.
Debbie, Denise, Dianna and Daisy were cloned using cells from the same adult sheep as Dolly, who became the first cloned mammal when she was born more than 20 years ago.
Concerns were raised when Dolly aged prematurely after being diagnosed with osteoarthritis at a relatively young age. She died aged six-and-a-half years old.
But the good health of Dolly's 'siblings' has given scientists hope that her poor health was not a result of the process that led to her creation.
The four cloned sheep, nicknamed the Nottingham Dollies, are part of a flock of 13 being monitored by researchers at the University of Nottingham.
The university's Professor Kevin Sinclair said of the findings, published in the journal Nature Communications:
One of the concerns in the early days was that cloned offspring were ageing prematurely and Dolly was diagnosed with osteoarthritis at the age of around five, so clearly this was a relevant area to investigate.
How does the cloning process work?
Dolly and her 'siblings' were created using a process called somatic cell nuclear transfer (SCNT)
An adult cell nucleus containing an animal's "signature" DNA is transferred to an unfertilised donor egg whose own nucleus has been removed
Electrical stimulus causes the egg to start dividing and form an embryo that is genetically identical to the donor of the adult cell
The Nottingham flock came about as a result of studies that were striving to improve the efficiency of SCNT.
Part of the cloning process involved "re-programming" cells so that they became blank slates with limitless potential, which helped advance work in stem cell science following Dolly's birth.
Despite technological advances in recent years, efficiency of SCNT remains low but there are several groups across the world working on this problem at present and there is reason to be optimistic that there will be significant improvements in future.