Lizzie Tenney, 13, paces across the living room, watching the sun disappear through the front windows of her Highland home.

• SHE POINTS a small, yellow ultraviolet meter toward the light and clicks: The magic number she's looking for is 10. Lizzie lives by what the box says. On this overcast November evening, it's unlikely much UV radiation pierces the clouds. The box thinks for a moment, then favors the girl: triple zeroes, the best possible reading.

She follows this routine four or five nights a week. A victim of xeroderma pigmentosum, an extremely rare genetic disorder that renders her skin unable to repair damage from UV rays, Lizzie and the sun have been at odds all her life. On a normal day, 15 minutes in the sun for Lizzie results in a burn most people would have after four hours of unprotected exposure.

The reason she waits so eagerly for the sunset can be pinpointed to one figure: the family dog. A fervid animal lover -- she also has birds and a snake and has been eyeing a stray cat hanging around the front doorstep lately -- Lizzie yearns for the time of day when she can walk the dog through the streets of her neighborhood freely. It's possible anytime, sure -- as long as Lizzie wears a long-sleeved outfit and specialized hood with a plastic visor to block the harmful sunlight. But the hood warps her vision, she said, and she much prefers to leave it behind.

"I get sad sometimes," she said. "I want to be like the other people."

The evening walks have been a long-standing tradition in the family, said her father, Steve. The family has learned different ways to cope with the problem since sunlight first lacerated Lizzie's skin as a baby. He said there's no other way for his daughter -- one of the "children of the night."

"The neighbors know who's going for a walk when the sun goes down," Steve said.

There are only about 250 known cases of XP nationwide and perhaps 1,000 across the world, said Michele Milota, executive director of the XP Family Support Group, who flew into Utah on Tuesday to help the Tenneys put UV screens on their windows. Experts -- and there aren't many in this field -- estimate about one in 250,000 babies is born with some variation of the disorder. Not by coincidence, most known cases are children; because the sun damage almost invariably leads to multiple bouts of skin cancer, the life expectancy of someone with XP is believed to be truncated by 30 years or more.

"When it first happens to you it's a death sentence," said Milota, a California native who has a daughter with XP. "They're missing DNA in their body to repair damage. Two minutes, 5 minutes [in the sun] -- it's all doing damage."

Lizzie's parents suspect it's precisely because of the rarity of the condition that it took so long to diagnose their daughter. Before this summer, Lizzie had spent her days on a revolving tour of dermatologists, most of whom told her to wear sunscreen and go home.

The Tenneys began another round of searching earlier this year when marks began to show up on Lizzie's cheeks and leg. But it wasn't until a visit to Alpine Dermatology's Dr. Cheryl Lee Eberting that answers began to emerge. Eberting had served an internship at the National Institutes of Health, one of the only U.S. organizations doing serious XP research, and was familiar with the telltale signs.

"She took one look at her face and said, 'Looks like skin cancer,' " Lizzie's mom, Kittie, recalled tearfully.

Further tests confirmed it was basal cell carcinoma. Dr. Ray Peterson of the Central Utah Clinic removed the three marks, but the likelihood is Lizzie will face many -- perhaps dozens -- of similar surgeries in the future. It remains to be seen whether other health complications associated with XP, including neurological decay and blindness, will surface.

"We don't know when it's going to come out, either. It could be 10 years, it could be 2 years," Steve Tenney said. "We just really are scared to death."

Still, it's possible to live a full life with XP, as Lizzie will tell you. In many ways, she's a typical teenager. She goes to school every day at the Renaissance Academy in Lehi. She loves Spanish, hates math. She exchanges messages with her friends over the Internet, primps her hair before photographs, and breaks into a wide grin when the phone rings with a call from the boy she's crushing on. She wants to be a preschool teacher when she grows up.

She even attended a summer camp this year set up by Milota especially for XP families. That gathering brought 27 kids from across the country to share experiences and find sympathy in one another's stories. The attendees got tickets to a concert by country star Brad Paisley, and Lizzie came home with a stuffed bear. She named it Xermy, after the disorder.

"It was awesome," she said. "I could play with them during the day."

Even the parents had fun, Kittie said -- and it was a tremendous experience to meet other parents coping with the same rare problem.

"We swam every night until about 3 a.m.," she said. "It was hard to describe. It was great, because they connected."

Back home, the Tenneys are still very much getting used to life with the new diagnosis. Nothing drastic has changed, they say, but the reality of XP is a heavy burden. That burden was in part lifted Wednesday by the generosity of strangers. Milota flew into town to coordinate a charity effort to apply UV screens to the windows of the Tenney home, the labor for which was donated by Alpine Window Tinting.

"We usually have a dark house," Steve said. "It's going to bring the light in."

Ace Stryker can be reached at 344-2556 or astryker@heraldextra.com.