There are fewer than 150 XP cases in the United States and under 1,000 known worldwide.
A few minutes of daylight or even fluorescent light can cause severe burns and irreversible damage. Caren Mahar first realized that something was wrong when her daughter Katie was 6 weeks old. The family, who lives in Poughkeepsie, N.Y., had taken Katie out into the back yard.
"We had her under the shade of a tree out in the back yard, and she just began screaming. And she had all these little red pin dots," she said. "We were watching each one of these bumps fill up into a boil, and it all became one big boil."
Katie had eight third-degree burns before she was a year and a half. It took 18 months of testing before doctors finally figured out that Katie had XP.
And Katie will have difficulties even in the best of circumstances, Caren Mahar said. "She will have skin cancer from the burns that she had as an infant," she said. "She will probably have eye cancers. Her prospects are horrible."
But the entire family is fighting to improve Katie's chances. The windows in their house are tinted. Katie goes outside only at night, often joined by her brothers and sister. For several years, she was a prisoner by day in her house, which was also her school.
This past year, Katie began going full time to a regular school. She still cannot linger in the daylight, but her classroom has sunproof windows so she has been free to carry on like any other second-grader.
"My favorite subject is snack - snack and recess," Katie said this summer.
Most children with this disease begin to fade, even die, at Katie's age. But Katie is free of both blemishes and tumors.
At first, Dan Mahar, Caren's husband and Katie's father, wouldn't accept that Katie had XP.
"I would get up and go to work in the morning, come back," e said.
"She would have curtains and blankets, everything all over the windows," Dan Mahar said of his wife. "I'd rip it all down, telling her she's overreacting. See, I thought it was a big con. I thought there's no such thing. It's like telling me you're allergic to water. For six months, we didn't speak at all," he said of their strife.
"We went through a pretty tough time," he added.
During that period, Katie got another burn, on her face.
"I had called the doctor, and he said, 'There's nothing you can do. Just give her some baby Tylenol,'" Caren Mahar remembered. "And so I'd go to give her the Tylenol and, of course, that cracks her mouth more. There was nothing as her parents that we could do to help her. And I - I walked outside. I said, 'I can't take this.'"
At that point, the Mahars decided to do something. They took out a second mortgage on their house and founded the Xeroderma Pigmentosum Society. Through the Internet, they now reach out to other families who also live in darkness.
"We did it so that Katie would never, ever be alone," said Caren Mahar. In July 1997, a group of XP families gathered in upstate New York. Families came from as far away as Italy and Russia.
Like Katie, some of the kids were in good health, but the disease was taking its toll on others with skin cancer and even neurological problems.
For the kids, though, the meeting - called Camp Sundown - was sunless fun: They got to play with other children at night.
With help from businesses, family, friends and neighbors, the Mahars opened the camp.
The campers' day began at 2 p.m. in the afternoon. As the kids played in the safety of the lodge, their parents shared the latest on the disease that rules their lives.
"It's an emotional thing that you share with someone who's going through the same experience," Caren Mahar said. When darkness came, Katie went to play outside, running around as if it were noon.
But these kids need more than fun at night. They need a cure. Researchers are working on that now. Research dollars for diseases this rare are hard to come by, however.
"Whether we'll solve some of the important barriers in the next year or the next five years depends on how we work and how well we're funded," said one of those researchers, James Cleaver of the University of California at San Francisco.
Dr. Cleaver was among the first to realize that XP patients' cells could not repair themselves and to identify the gene that causes XP. As with other genetic diseases, gene therapy may hold the cure down the road.
"It's very frustrating because we've been told that a cure is obtainable," Caren Mahar said. "That's the part that hurts the most."
"When you reduce that beautiful child, when you reduce her life to a matter of money, it turns my stomach beyond words," she added.
So Caren Mahar stays on a nonstop crusade.
In the five years since the Mahars founded the XP Society, they've raised about $140,000. They are told it will take roughly $800,000 to pay for research, which doctors say would likely result in a cure for this rare genetic disease.