Happy 23rd Birthday Dolly!

Well, sort of. Technically, she was born July 5, 1996, but it was on this day in 1997 that scientists at the Roslin Institute in Scotland announced the birth of Dolly, a female sheep who was the first mammal to have successfully been cloned from an adult cell. She was the only lamb that survived to adulthood from 277 attempts.

The funding for Dolly’s cloning was provided by PPL Therapeutics —a Scottish biotech firm near the University of Edinburgh, where the institute is based—and the British Ministry of Agriculture.

Dolly was born the summer before and had three mothers: one provided the egg, another the DNA, and a third carried the cloned embryo to term. She was created using the technique of “somatic cell nuclear transfer”, where the cell nucleus from an adult cell is transferred into a developing egg cell (called an unfertilized oocyte) that has had its cell nucleus removed. An electric shock stimulates the hybrid cell to divide, and when it develops into a blastocyst (which will eventually form the embryo) it is implanted in a surrogate mother.

Dolly lived only about half as long as her breed, leading some to speculate that her cloning had something to do with it. However, an analysis of her DNA found no anomalies, and her death by lung disease is particularly common for sheep kept indoors (Dolly had to sleep inside for security reasons). None of Dolly’s six children—the result of conventional mating with another sheep—bore any unusual defects of properties. As of 2016, scientists reported no defects in thirteen cloned sheep, including four from the same cell line as Dolly.

Dolly’s legacy has far outlived her, and will likely continue to into the 21st century. She quickly paved the way for the successful cloning of other large mammals, including pigs, deer, horses, and bulls. Making cloned mammals was initially highly inefficient, but by 2014, Chinese scientists reported a 70–80% success rate cloning pigs, while in 2016, a Korean company, Sooam Biotech, was producing 500 clones embryos a day.

As recently as 2018, a primate species was successfully cloned using the same method for producing Dolly: Two identical clones of a macaque monkey, Zhong Zhong and Hua Hua, were created by researchers in China. Just last year, Chinese scientists reported the creation of five identical cloned gene-edited monkeys, using the same cloning technique as for Dolly and Zhong Zhong and Hua Hua.

There have also been attempts to clone extinct species back into existence. The most famous attempt was in 2009, when Spanish scientists announced the cloning of the Pyrenean ibex, a form of wild mountain goat, which was officially declared extinct in 2000. Although the newborn ibex died shortly after birth due to physical defects, it is the first time an extinct animal has been cloned, and may open doors for saving endangered and newly extinct species by resurrecting them from frozen tissue.