"Nuclear transfer involves inserting a fully developed cell -- in Mitalipov's study, the cells came from the skin of fetuses -- into the nucleus of an egg, and then manipulating the egg to start dividing, a process that normally only occurs after it has been fertilized by a sperm.

After several days, the ball of cells that results contains a blanket of embryonic stem cells endowed with the genetic material of the donor skin cell, which have the ability to generate every cell type from that donor.

In Dolly's case, those cells were allowed to continue developing into an embryo that was then transferred to a ewe to produce a cloned sheep. But Mitalipov says his process with the human cells isn't designed to generate a human clone, but rather just to create the embryonic stem cells. These could then be manipulated to create heart, nerve or other cells that can repair or treat disease.

"I think this is a really important advance," says Dieter Egli, an investigator at the New York Stem Cell Foundation and Columbia University. "I have a very high confidence that versions of this technique will work very well; it's something that the field has been waiting for."

Egli is among the handful of scientists who have been working to perfect the technique with human cells and in 2011, succeeded in producing human stem cells, but with double the number of chromosomes."