Twin rhesus macaques Mito and Tracker were named after the dye used to detect mitochondria, the DNA of which can be defective. (Courtesy Oregon National Primate Research Center)

An experimental procedure that someday may enable women to avoid passing certain genetic diseases on to their children has gained an early success, with the birth of four healthy monkeys, scientists reported Wednesday.

The technique still faces safety questions and perhaps ethical hurdles, but an expert called the work exciting.

The experiment, which involved transferring DNA between eggs from rhesus macaques, was described Wednesday on the web site of the journal Nature by researchers from the Oregon Health and Science University.

Someday, the technique may be used against diseases caused by inherited defects in the "power plants" of cells, called mitochondria. These conditions are uncommon and unfamiliar to most people, such as Leber hereditary optic neuropathy. Roughly one person in every 4,000 or 5,000 either has one of these mitochondrial diseases or is at risk for one.

Symptoms of these potentially fatal illnesses include muscle weakness, dementia, movement disorders, blindness, hearing loss, and problems of the heart, muscle and kidney.

An egg contains the vast majority of its DNA in the nucleus, but mitochondria contain DNA elsewhere in the egg. So if a woman has a disease caused by defects in the mitochondrial DNA, the new technique might someday make it possible for her to pass on her normal DNA from the nucleus but not the flawed DNA from the mitochondria.

To allow for that, doctors may transplant nucleus DNA from the eggs of such women into donor eggs that have healthy mitochondria. The donor eggs would have had their own nucleus DNA removed. After test-tube fertilization, this egg would in theory produce a baby without mitochondrial defects. (Fathers do not pass along their mitochondria.)

Researcher Shoukhrat Mitalipov said more safety studies must be done in monkeys. He noted that the technique would face regulatory hurdles for human studies because it would change the DNA inherited by future generations, an idea that has long provoked ethical concerns.

Douglas Wallace of the University of California, Irvine, an authority on mitochondria who wasn't involved in the federally funded experiment, said the results were exciting and the technique is "potentially very interesting."

However, "there are safety issues that are going to need to be addressed before one could think about it in humans," Wallace said.