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In a darkened lab in the north of England, a research associate is intensely focused on the microscope in front of her. She carefully maneuvers a long glass tube that she uses to manipulate early human embryos.

"It's like microsurgery," says Laura Irving of Newcastle University.

Irving is part of a team of scientists trying to replace defective DNA with healthy DNA. They hope this procedure could one day help women who are carrying genetic disorders have healthy children.

"We are talking about conditions for which there is currently no cure," says Dr. Doug Turnbull, a professor of neurology at Newcastle University who is leading the research. These mitochondrial diseases are caused by hereditary defects in human cells.

"Mitochondria are like little power stations present in all our cells," Turnbull says. These power stations provide the energy that cells need. If the mitochondrial DNA is defective, the cells don't work right. The cells in effect run out of energy.

"I see the anguish of the families in every clinic that I do," says Turnbull. The severity of the disease...