Each day brings Jenn McNary another dose of hope and heartache as she watches one son get healthier while the other becomes sicker.

Both of McNary's sons were born with Duchenne muscular dystrophy. Max, 11, is receiving an experimental therapy that appears to be making him better, while 14-year-old Austin is slowly dying.

Austin was too sick to be included in the clinical trials for a promising new drug called Eteplirsen. “He can’t get into a chair, out of his wheelchair, into his bed and onto the toilet,” McNary told NBC’s Janet Shamlian.

Max, however, was exactly what researchers were looking for. He was put on Eteplirsen, and now he's back to running around, climbing stairs and even playing soccer.

“It’s a miracle,” McNary said. “It really is a miracle drug. This is something that nobody ever expected and he looks like an almost normal 11-year-old.” She and her sons were first profiled last month by the Burlington Free Press.

Eteplirsen is designed to partially repair one of the common genetic mutations that causes DMD. Even a partial repair may enough to improve life for boys struck by the condition, which results from a defect in the dystrophin gene.

That gene resides on the X chromosome, which is why only boys end up with DMD. Boys get one X and one Y chromosome. Girls get two copies of the X chromosome — one from their mother and one from their father — so even if they inherit a defective copy from their mom, they get a healthy one from their dad. Although they won’t suffer symptoms, girls wind up with a 50 percent chance of being carriers for DMD.

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Once McNary’s boys were diagnosed with DMD, she got tested. To her surprise and confusion, doctors told her that she didn’t carry the genetic mutation for DMD. “It was actually pretty shocking because I had been told they couldn’t both have it because I’m not a carrier,” McNary told Shamlian.

As it turns out, it is possible for a new genetic mutation to show up just in the mother’s eggs even if she doesn’t carry the mutation herself, according to DMD.org.

A working version of the gene is essential because it contains the codes for a protein called dystrophin, which protects the muscles from fraying as they contract and relax. The mutation in boys with DMD leads to a non-functional version of the protein and as the boys get older, their muscles start to deteriorate.

One of the most common mutations in the dystrophin gene is a tiny garbled section of DNA.

Think of the gene as a series of instructions laid end to end on a strip of tape. If an instruction in the middle is garbled, then the remaining instructions can’t be read and the protein comes out very flawed.

Researchers suspected that a chemical “patch” could be made that would cover over the garbled section of code. With the patch in place, cells might be able read the instructions from beginning to end, minus the little bit that had been patched over. The hope was that even if the resulting protein wasn’t perfect because it was missing a tiny bit of instructions, it might be good enough to protect a boy’s muscles.

Eteplirsen is designed to be one of those patches. And so far, it looks like it’s doing what researchers hoped it would.

The medication is currently being tested at Nationwide Children’s Hospital in Columbus, Ohio.

“Everything I can tell indicates that this is a winner,” said Dr. Jerry Mendell, director of the center for gene therapy at Children’s. “It’s a game-changer.”

And while the news seems to be very good for Max, the prognosis for Austin is dire because he doesn’t qualify for any clinical trials and until the Food and Drug Administration approves it, no one can receive Eteplirsen outside of an approved clinical trial.

Without the drug, “in the next six or seven years, he will die,” McNary said. "But we hope that doesn’t happen.”

McNary has become an advocate and is now asking the FDA for accelerated approval of the drug so that Austin and other boys might have access to it in a year instead of the usual four. The FDA tells NBC that it is currently reviewing the application.

Austin hopes his mom wins the battle.

“It’s really hard because I know I’m getting worse and he’s getting better,” he said.