One Ray of Sunshine Can Turn Katie’s Day Into a Nightmare : Health: Three-year-old suffers from a rare genetic disorder that makes body unable to process ultraviolet light. In dark, she plays.

When the sun sets, Katie Mahar comes out to play.

Climbing on her swing set, the 3-year-old looks the picture of health. She shouts at her brother and stands on a swing. She smiles and whips her dirty blond hair around. Only the dark sky gives a clue to Katie’s condition:

Daylight can kill her.

Katie’s life depends on hiding from ultraviolet rays. Even sunbeams slipping through a window blind can hurt her. Sunscreen is applied up to eight times a day. Visits to the doctor are made at night in a car with tinted glass. Once there, she wears a towel on her head and gloves to protect her from fluorescent lights.

Her parents teach their daughter about her special friend, Mr. Moon, and about her enemy.

“The sun’s a monster,” Katie confides. “I don’t like the sun.”

Katie suffers from xeroderma pigmentosum--or XP--a rare genetic disorder that makes the body unable to process ultraviolet radiation. More than half the children like Katie contract skin cancer before their 12th birthdays, and many die.

So Dan and Caren Mahar are in a sort of race against time. They’ve turned the rhythm of day and night on its head by sealing sunlight from their home. At the same time, they’ve started an XP foundation in the hopes of helping others like Katie and hastening a cure.

A quiet couple living on Dan’s salary as a mailman, they hope exposing Katie to some limelight now will one day allow her to walk in the sun.

“When you see someone who’s healthy and alive and has a drive for life and you know what’s waiting in the wings for her, well, we were faced with two choices,” Dan said. “We could either remain private and do absolutely nothing and pray--and praying is good--or we could make a . . . uh . . . “

” . . . A big blast,” Caren said.

“As big as we can, to say, ‘Hey, look at her!’ ” Dan said.

Katie is cute and bubbly with dark brown eyes often hidden, necessarily, behind sunglasses. Until she was a month old, the Mahars weren’t aware of anything extraordinary about their fourth child. Then they placed her under a shade tree on a warm spring day.

Within seconds, Katie was screaming and little pin dots formed on her face. Inside the house later, the dots turned to boils that hardened into scabs.

“Inside, her eyes were still coming out through this big happy baby. But every time she’d try to smile, it would crack the skin and then she would cry into it,” Caren Mahar said. “She was actually shaking in the pain of it all.”

The Mahars were horrified, but thought all was well after Katie healed. Then the plague-like reaction occurred again when Katie was carried outside briefly for a picture. More episodes followed. One time, it happened indoors as sun streamed through a kitchen window.

“The sun came in, hit her and we stood here, watched it like it was time-lapse photography. You could watch, literally, the blisters come straight up her skin and up her arms,” Dan said.

Tests followed for diseases like lupus and leukemia, and Dan and Caren were thankful when every test came back negative. After months, doctors finally determined last fall that Katie had XP.

The chances of a child being born with the disorder are about one in a million, and only about 250 people are believed to be alive with the condition in the United States, said Dr. Kenneth Kraemer, a research dermatologist with the National Cancer Institute.

Both parents must carry the recessive gene that triggers it. Even then, the chances of the Mahars having a child with Katie’s condition were one in four. Their previous three children had no signs of the genetic defect.

The Mahars decided to do all they could to prolong Katie’s life, painfully aware of what their choice meant.

Garbage bag shades on the first-floor windows of their Hudson Valley house block the view of nearby woods. Katie can’t watch a flower track the sun or her siblings get off the school bus. Only incandescent bulbs are allowed in the house, because they produce less ultraviolet radiation.

Books like Beatrix Potter’s “Tom Kitten” are read to her, describing a daylight world she can see only in the pictures or on television. She can’t visit zoos, playgrounds or toy stores. Normal childhood pursuits like friendships and school are logistics problems yet to be solved.

The Mahars’ other children, ages 8 to 5, play outside with Katie at night, and the swing set will be moved into the garage this summer.

Caren, 34, passed up job offers to devote herself to Katie, and Dan, 36, spends time with her when he can. Last December, he drove her around town in the quiet before dawn so she could see the Christmas light displays.

With proper protection from ultraviolet rays, XP sufferers can live to adulthood. Still, the Mahars’ extraordinary efforts might not be enough.

Katie’s previous exposure to the sun might have been enough to cause future problems. Her condition also gives her a heightened chance of developing a neurological disorder. She could wake up tomorrow, for example, going blind or with her mental development forever arrested.

Researchers are making progress in finding a treatment for XP, such as a topical cream that would deliver proteins to protect from the sun, said Dr. James Cleaver, associate director of the Laboratory of Radiobiology and Environmental Health at the University of California, San Francisco.

But with a cure maybe 10 years away, the Mahars hope telling Katie’s story will speed the process. Caren said she wants to raise money for research and education, as well as for a year-round “night camp” for XP kids.

The Mahars hope their daughter will live long enough to understand their reasons for the exposure.

“This is my daughter,” Dan said. “If we can buy her another year, another six months, we have to do anything to prolong her life.”

“We are literally petrified of this . . . but what would you do?’

The Xeroderma Pigmentosum Society is reachable by writing 57 Sleight-Plass Road, Poughkeepsie, N.Y. 12603, or calling (914) 473-4735.