Oregon National Primate Research Center scientists say they’ve reached the long-sought goal of cloning monkey embryos and extracting stem cells from them, a potentially major step toward doing the same thing in people.

In cloning to obtain stem cells, DNA from an adult animal is inserted into an unfertilized egg. The egg is grown into an early embryo, from which stem cells are extracted. These stem cells, and the tissue that develops from them, will be a genetic match to the source of the DNA.

The idea of doing this in people is controversial because the embryos have to be destroyed to obtain the stem cells.