How did Ian Wilmut and Lawrence Smith produce cloned lambs in 1989?

In 1989, British biologist Ian Wilmut and graduate student Lawrence Smith produced four cloned lambs at the Roslin Institute using a technique known as embryonic cell nuclear transfer.1

In this process, the nucleus from an embryonic stem cell was inserted into an egg cell that had been enucleated (its own nucleus removed). Wilmut and Smith later discovered that the success of these clones was accidental; they realized that the specific stage of the cell cycle at the time of the transfer was the critical factor determining whether the experiment would succeed or fail. This early research was foundational, as it highlighted the importance of cell cycle synchronization in nuclear transfer, a principle that later became instrumental in the successful cloning of Dolly in 1996.21