A life may have been saved by a four-year-old little girl and she doesn't even know it.

Bella Stauffer gave her twin brother, Ryan, her bone marrow. Without it, Ryan would die.

We told you about Ryan Stauffer a few months ago when Ryan was diagnosed of a rare genetic disease called Adrenoleukodystrophy. Now, we tell you about his possible cure.

The day begins with an early morning for 4-year-old Bella. She arrives at Children's Hospital in Denver, sleepy eyed with her stuffed giraffe for comfort. She has no idea she will give her twin brother Ryan, the best gift she'll ever give.

"We told her we're taking her healthy cells and giving then to Ryan," Chrissy, Ryan and Bella's mom said.

At just 4-years-old, Ryan is very sick. He has Adrenoleukodystrophy, or ALD, a rare genetic disease.

A bone marrow transplant is the only thing that can save his life. Without it, "This would probably progress over the next 4 to 5 years, so much so, that when you saw him as say a 10-year-old he would be what you would classify as severely retarded and probably die over the next decade," Dr. Ralph Quinones, Ryan's doctor, explained.

His sister Bella can keep that from happening. Her bone marrow is a match. Bella starts her journey getting checked out by the nurses. She doesn't like being poked and prodded, but it's not long before daddy is carrying her to the operating room. This is where the masked doctors will take the marrow out of her hip bone, for her brother. Once she's asleep, the doctors make two holes, and little, by little, they take the marrow they need to save Ryan.

"It will save the quality of his life, ultimately it will save his life as well," Dr. Quinones says.

While his sister is in the operating room, Ryan sits in isolation. He's had two weeks of intensive chemotherapy.

"Doses of chemotherapy that are so high, they will permanently poison or destroy his bone marrow so that the new bone marrow has a place to move into and live," Dr Quinones says.

Because Ryan's bone marrow is gone, Ryan has to stay in a germ free environment, because Ryan has no immunity, a simple cold virus could kill him.

Once the bone marrow has been taken out of Bella, it goes to the processing lab, where all of the red blood cells will be taken out, so that Ryan gets the best possible product.

Once the bone marrow is treated, Ryan will get what was taken from his sister. It will be given to him through an IV. He'll barely feel it.

"If everything goes well, her bone marrow will grow, start creating new, healthy cells for Ryan, and cure him," Dr. Quinones says.

She'll have just a few small scars,

"In the future she'll be proud of the scars," Chrissy says.

In the future, she may understand, that for sister and brother, this is a life changing day.

It will take several weeks to see if Ryan's body accepts Bella's bone marrow. Parents and doctors have fingers crossed.

Then it will take another six months or more for Ryan to be healthy enough to go home.

If everything works, one year from now, Ryan will go to kindergarten, as a normal and healthy little boy.