Sweet and optimistic, Katie Mahar is a typical 10-year-old girl.

The Craryville, N.Y., resident is the youngest of four children. She jumps on her trampoline, listens to the Backstreet Boys and plays with her kitten, Pumpkin.

She also has a disease called xeroderma pigmentosum, a genetic disorder also known as XP.

Sufferers can't tolerate ultraviolet light, a form of solar radiation not visible to the human eye.

Suk-Hee Lee, a cancer researcher at the Indiana University School of Medicine, studies DNA gene repair. He defines XP as "an extreme sensitivity to ultraviolet exposure." XP patients' skin cells, he explained, can't repair themselves after UV exposure.

According to Lee, Katie is part of a small group with the disease -- 250,00 to 500,000 people worldwide. Some are born with it, while others acquire it. Katie's disease is the rarest and most severe light-sensitivity disorder.

In the past, this disease was rarely diagnosed. In 1997, a registry was begun with 250 XP patients. Today, it lists about 3,000 patients worldwide, Lee said.

Luckily, Katie's parents discovered her condition when she was 2 years old. It happened when she was outside on a swing.

"I got these little red dots on my hand and all over my face. And then they boiled up like big blisters. . . . So it really hurts, and we have to go to an emergency room, which makes them hurt more because it has so much more fluorescent lighting," she said in a phone interview.

The severity of XP varies. Some patients can go outside with minimal protection; others can end up in the hospital simply by stepping outside.

Besides the blisters Katie developed, other symptoms include lesions, discoloration, inflammation, premature aging of the skin, eye problems, neurological abnormalities and a predisposition to all forms of cancer. Unless sufferers stay away from the sun, multiple skin cancers could result. Eyes are equally susceptible; small amounts of ultraviolet light can damage the corneas.

Precautions inside and out

To maintain a safe level of lighting inside the Mahars' house, precautions must be taken.

"The windows are tinted around the house so I can walk around it without wearing all my stuff on," Katie said.

Aside from tinted windows and blinds or drapes, interior lighting also must be restricted. Only low-wattage, incandescent bulbs may be used in the house, and the family checks for ultraviolet rays using a light meter.

To spend time outside, Katie's "stuff" includes SPF 45 sunscreen, light-proof fabrics, an eye shield and melanin sunscreen.

"I have to get lotion on. I have to lay this shield over my face so I don't get hit by the sun, and I also have to wear sunglasses, gloves and a hat and a jacket. . . . You know how you get a sunburn. I get it 200 times that."

Medication is an important part of treatment.

"I have to take a big canteen that holds pills. I have to take six of them each and every day at school," Katie said. "In all the seasons except for winter, I have to take (my medication) three times a day." Given the disease's rarity and relatively recent discovery, it is not yet known how to provide the best protection.

Research has found two forms of XP therapy. In protein therapy, a missing protein is introduced directly into the cell; gene therapy changes the cell's DNA content so it can create the missing protein. Although gene therapy is at this time nearly impossible, Lee hopes it soon will provide the cure.

Many would assume school would be difficult. But Katie said she has no problem with friends or schoolwork.

"They treat me just the same as they treat each other, just I'm the only one that has XP in my school. . . . I play at school with everyone, and sometimes I'll have them over to sleep over, and we'll go on the trampoline, play baseball, basketball and all sorts of stuff outside and inside," she said.

The only difference between Katie and her classmates' school day is recess; she plays inside.

"Sometimes I will just read a book if I'm really bored," she said.

Adjusting schedule

Unlike ordinary children who spend an average of one to two hours outside each day, Katie sleeps during the day and plays outside at night. Katie's schedule naturally allows for a later bedtime, which she views as a great advantage.

"The best part about having XP is that even though my brothers and my sisters go outside in the daytime, I still get my chance to go outside during the night. So either way, I'll get some fresh air outside. . . . In summer, I'll stay up till 7 a.m., then I'll go sleep and wake up at 3 in the afternoon."

During the summer, Katie participates in Camp Sundown in Poughkeepsie, N.Y. It's a weeklong camp for children with XP. The camp runs from the unusual hours of 9 p.m. to 5 a.m. and involves close to 300 families. Nighttime events include field trips, hayrides and water-balloon fights. Indoor activities and crafts are open for the campers during the day.

In 2003, the camp plans a permanent location in Claverack, N.Y. The future retreat building, currently under construction, includes 12 bedrooms, a kitchen, dining room, activity room and an indoor swimming pool.

Katie's parents, Dan and Caren Mahar, founded the XP Society, which runs the camp and has a Web site (www.xps.org/) and support group.

While the hope for a cure is far off, Katie is content with life.

"They are trying to find a cure for XP so that we can go outside in the day and enjoy it. . . . Sometimes I really want to get the disease out of my system, but other times I'm just fine the way I am.

"I'm just like everyone else, except I have a disease."

ASSISTANT EDITOR: Evan Daniluck, 15.

REPORTERS: Stephanie Ray, 11, and Kelsey Ebbert, 14.