A team of New York City scientists has created human stem cells by implanting the nucleus of an adult skin cell into an egg cell—a feat otherwise known as cloning.

The advance represents a major success in human stem cell research. Researchers across the globe have been trying to clone human cells for at least a decade, because such cells have great potential value for both medical research and cell replacement therapies. But the procedure—which has become routine in other mammals—has proven extremely technically challenging.

"This work now demonstrates for the first time that the human egg has the ability to turn a specialized cell into a stem cell," said lead researcher Dieter Egli in a telephone news conference on Tuesday. The research appears in today's issue of the journal Nature.

In nuclear transfer, or cloning, the DNA of an adult cell is injected into an egg, which typically has its own DNA removed. The egg then develops into an embryo, from which embryonic stem cells can be extracted. (The need to create and destroy a human embryo has made the procedure highly controversial, and such research cannot be funded with federal grants.)

When Egli and collaborators tried the standard approach, the resulting cell would divide six or 10 times and then die. So instead, they left the egg's DNA intact. The resulting cells have three sets of chromosomes—two from the cell donor and one from the egg. That makes the cells unsuitable for therapeutic use, but it may also quell some of the controversy surrounding human cloning, since the embryo is incapable of growing into a human. However, Egli believes he will be able to make cloned stem cells with the normal two sets of chromosomes.

Most human embryonic stem cells used today in research were created from discarded human embryos. The cells are pluripotent, meaning they can differentiate into any type of cell. Cloning has the capacity to create stem cells that are genetically matched to the cell donor; such cells would be ideal for use in cell therapies, because they would not trigger an immune reaction. And they are useful in research, allowing scientists to study cells created from people with specific diseases. 

In 2006, Japanese scientist Shinya Yamanaka developed an alternative method to create genetically matched stem cells called induced pluripotent stem cells, which used chemicals rather than a human egg to turn back the cellular clock in an adult cell. Work with embryonic stem cells—controversial because of the use of embryos—fell out of favor, as most researchers jumped on Yamanaka's technique.