Inside the experimental treatment considered for baby Charlie Gard

— -- The U.S. doctor in London to examine Charlie Gard, an 11-month-old boy suffering from a critical illness that has damaged his brain and rendered him unable to breathe on his own, could recommend an experimental treatment -- one which may or may not improve his outcome.

A U.K. judge extended invitations to Dr. Michio Hirano, chief of the division of Neuromuscular disorders and a professor of neurology at Columbia University in New York City, as well as a doctor from the Bambino Gesu hospital in Rome who has not been named, after evidence on a new experimental treatment was presented in court last Thursday.

The doctors were each given an "honorary contract" by the Great Ormond Street Hospital to examine the baby, use its facilities, review medical records and speak with his doctors and parents.

But questions remain about whether the experimental treatment would help Charlie because it has never been tested for his specific condition.

A disease that affects organ function

Charlie suffers from a form of mitochondrial depletion syndrome, a rare genetic disease that causes progressively increasing muscle weakness that leads to organ failure and becomes life-threatening within a few years. Though he is less than a year old, the baby has been on life support for several months.

He has a rare mutated gene, the RRM2B gene. The mutation impairs his production of nucleosides, a sub-part within the DNA of his mitochondria. Mitochondria, which are called the "powerhouses" of the cells, are responsible for creating the energy to sustain cells and therefore life.

Nucleosides are required to help produce healthy mitochondrial DNA, which are necessary for mitochondria to produce energy for cells, including the energy-hungry muscles. Healthy muscle tissue is, in turn, required for organs to function normally.

That's why a lack of healthy nucleosides will lead to failure of many organs, according to the National Institutes of Health, including those required for breathing.

Taking a risk on the medicine

The experimental therapy that has been suggested for Charlie would be an oral medicine that aims to deliver the correct type of nucleoside to improve the function of his mitochondrial DNA.

The medication has been tested on mice and a small number of people with a different mitochondrial condition, some of whom have shown measurable improvement.

But the drug has never been tested on people with Charlie's specific condition. As a result, the baby's response to the experimental treatment is difficult to predict.

Encouragement from another family

ABC News' "Nightline" reported on an American family from Baltimore who understands the trying circumstances for Charlie’s parents and recommends the experimental treatment.

Art and Olga Estopinan’s 6-year-old son Art Jr. was diagnosed with a similar type of mitochondrial depletion syndrome when he was just over a year old. Like Charlie, doctors had told his parents there was little chance of survival.

Five years ago, Art Jr. became the first child in the U.S. to receive the treatment and Hirano was the doctor who administered it.

"We were told there was no hope and no cure," Olga Estopinan told "Nightline." "He was terminal and the average age for these children was 3 years old."

The Estopinans said little Art Jr. went from being on the brink of death to gaining back some motor functions. He can now find and play movies on an iPad and say "Mommy," "Daddy" and a few other words.

The medicine has not "cured" Art Jr. He requires round-the-clock care, receives three to four treatments per day, eats through a feeding tube, uses a motorized wheelchair and needs help moving his limbs, "Nightline" reported.

Despite those challenges, Art Jr.'s family said it has changed their son's life and recommend it.

"With all due respect to [Charlie’s doctors,] I encourage those doctors to educate themselves," Art Estopinan told "Nightline." "See how these experimental medications will create the end signs that little Charlie needs so he can get stronger like my son."

ABC News' Dan Childs, Mike Trew, Bianca Seidman, Joseph Simonetti, Linzie Janis, Sally Hawkins and Candace Smith contributed to this report.