Dolly the sheep stunned the scientific world by proving it was possible to clone a mammal from an adult cell.
Until Dolly's birth 20 years ago on July 5, 1996, it was widely believed that adult cells were genetically "fixed" in a way that would not allow cloning.
Previously scientists produced clones of frogs, mice, cows and sheep using cell nuclei taken from early stage embryos.
But the team at the University of Edinburgh's Roslin Institute led by Professor Ian Wilmut made history by cloning Dolly from a cell taken from the udder of a six-year-old Finn Dorset sheep.
Dolly was the sole survivor of 277 cloning attempts during which 29 early embryos were implanted into the wombs of 13 surrogate mothers.
Her birth triggered a revolution in genetics and stem cell science.
The technique used to create Dolly is called somatic cell nuclear transfer. Greatly simplified, it involves transferring an adult cell nucleus containing an animal's "signature" DNA 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.
At the time of Dolly's birth there was wild speculation about the possibility of human clones. Dolly's most important legacy related to stem cell research rather than cloning.
Part of the cloning process involved "re-programming" cells so that they become blank slates with limitless potential.
This led directly to the development of induced pluripotent stem cells (iPS cells) - stem cells produced by genetically reprogramming ordinary adult cells, such as those in the skin.
Like stem cells plucked from early stage embryos, iPS cells have the potential to transform into any kind of cell in the body.
The development of iPS cells raises the prospect of personalised stem cell treatments, derived from a patient's own cells, which will not be rejected by the immune system.
Dolly mated and produced offspring, showing that cloned animals can reproduce.
She was put to sleep at the young age of six-and-a-half, suffering from arthritis and a virus-induced lung tumour.
Analysis of her DNA suggested that she had aged prematurely as a result of being cloned from a sheep that was already six-years-old. Under normal circumstances, sheep can live to 11 or 12.
