NBC News - At 15, Hayley Mogul lacks the fine motor skills needed to write. Her sister Bari is 9 and still eating baby food.

There's no cure for their rare disorders, caused by unique genetic mutations. But for once, there's an advantage to having conditions so rare that drug companies cannot even think of looking for a cure. The sisters are taking part in a whole new kind of experiment in which scientists are literally turning back the clock on their cells.

They're using an experimental technique to transform the cells into embryonic form, and then growing these baby cells in lab dishes.

The goal is the get the cells to misfire in the lab in just the same way they are in Hayley's and Bari's bodies. It's a new marriage of genetics and stem cell research, and represents one of the most promising applications of so-called pluripotent stem cells.

"One day these two girls will probably change the face of medicine as we know it," said their father, Steven Mogul.

Steven and Robyn Mogul don't understand why both their daughters ended up with the rare mutations, which cause a range of neurological and metabolic problems.

"We have been tested," said Mogul, a 45-year-old wealth manager living in Chicago. "We don't have any mutations, and there are no developmental issues. We have no idea how it happened. "

The girls need special schooling and physical therapy. They must wear diapers, and when they get a cold or the flu, they can develop dangerously low blood sugar. "When the kids get sick, get colds or flu, we have to get them to the hospital," Mogul said.

Hayley, 15, has a mutation in a gene called RAI1, which can cause Smith-Magenis syndrome. The syndrome affects 1 in 25,000 people and can disturb sleep patterns, cause obesity and behavioral issues. But Hayley's mutation is unique and puzzling. Bari, 9, has an RAI1 mutation and a similarly unique mutation in the GRIN2B gene, which can cause learning disabilities.

"Bari doesn't talk," Mogul said. "She walks around, she gets around and lets you know what she wants. She is eating baby food and she is drinking from bottles."

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'Something positive for humankind': Girls lend cells to genetic study