New research suggests Dolly's cloning process didn't create health problems.
In 1997, scientists announced they'd created a healthy sheep cloned from another ewe's mammary gland cell. Two decades on, the technique is being refined and applied to new challenges.
It took years of attempts before scientists were able to clone a mammal from an adult cell. And with that success came plenty more questions.
Dolly the sheep died young with osteoarthritis, but new evidence shows early-onset diseases aren't inevitable for clones.