Why was Dolly a milestone in the history of cloning?

Dolly the sheep, born in 1996, was a landmark achievement in science because she was the first clone of an adult mammal.123

Before Dolly, it was widely presumed that cell differentiation in adult animals was irreversible, meaning adult cells had lost the totipotency required to create a new, viable organism. While scientists had previously generated clones from undifferentiated or partially differentiated embryonic cells, the successful production of Dolly by British biologist Ian Wilmut and his team at the Roslin Institute proved that a fully differentiated adult cell—in this case, from a mammary gland—could be reprogrammed to guide embryonic development.34

This breakthrough utilized a technique known as somatic cell nuclear transfer (SCNT), which involves transferring the nucleus of a somatic cell into an enucleated egg cell. Dolly’s birth dispelled decades of scientific skepticism regarding adult cloning and fundamentally advanced our understanding of cellular mechanisms, nuclear reprogramming, and stem cell research.143