Monica Anderson, 9, literally is living in a body cast of her own bone.

In the last year Monica, the victim of an extremely rare genetic disorder, has lost the ability to move her elbows, knees and shoulders because the ligaments and tissues around them have turned to bone.

A sheet of bone has grown around her neck, encasing it, her doctor says, "like a tin can." When she breathes, she cannot expand her chest or rib cage because ligaments and tissues connecting her ribs also have turned to bone. Bone now connects her jaw with the top of her spinal cord, making it difficult for her to open her mouth or to eat.

Her condition, which is progressive and keeps her in constant pain, could get worse. Doctors do not know if life for Monica is a matter of months or years.

Monica is a patient at the clinical center of the National Institutes of Health. Despite the emotional trauma of her relentless and debilitating disease and the prospect of death, she and her parents have accepted it with a degree of serenity and are pleasant and cheerful in dealings with doctors, nurses and fellow patients.

"She is a super child and her parents are extraordinary," says Dr. Michael Zasloff, Monica's doctor and a senior investigator with the arthritis institute at NIH. "She's pleasant and she's not at all bitter.She's made our job easier."

As her condition has progressed, Zasloff said, Monica has become increasingly vulnerable to infection, such as pneumonia, and further ossification could impair the functioning of her vital organs.

To her schoolmates at St. Francis Xavier School, 2815 O St. SE, Monica has become a symbol of the power of the human spirit, according to the school principal, Sister Mary Teresa.

"She has touched the lives of every student in our school," said Sister Mary Teresa. "Nobody seeing her condition can ever take life for granted. She has preached a sermon more powerful than those we have heard from many pulpits."

Monica's mother, Lydia Anderson, says, "I just feel if my husband and I keep our composure, it will make Monica's life much easier," she said. "I try to keep myself together in order to keep her together."

When Monica, an only child, entered the clinical center March 14, her mother went on unpaid leave from her job as a nursery school teacher to live with her daughter at NIH. Nights she sleeps on a rollaway bed in Monica's room.

Monica's father, John Wesley Anderson, a supervisor of fingerprint examiners with the FBI, visits his wife and daughter every night after work, then drives home to Suitland with a load of laundry that he washes and brings back to NIH the next night. He and his wife have dinner together each evening at the NIH cafeteria.

"No one person and no one medical center knows enough about this to understand what's happened and what's going to happen," says Zasloff. "It's so rare that everybody's experience is anecdotal. There just aren't enough people who have a feeling about the course of this disease. Monica could live another few months or she could live to be 60."

Monica is afflicted with myositis ossificans progressiva, which Zasloff describes as a "genetic disorder of exceeding rarity." About one in 500,000 births has myositis ossificans, Zasloff said, but he said the disease can go into remission and persons afficted with it have been known to live into their 70s.

"The area around Monica's muscles develops into bone," Zasloff says. "It can involve any muscle. It is a condition which is progressive and it results in the formation of bone throughout her entire body."

Monica was born with myositis ossificans, but it was not diagnosed until she was 3. Only within the last year has it flared up.

"Until about a year ago, things were more or less quiet in Monica's body," said Zasloff. "When we saw her last year, she was walking. She had the use of her hips, elbows and shoulders."

When Monica was born, the big toes on each foot were bent inward and she had short thumbs, often indicators of myositis ossificans, but since the disease is so rare, her doctors made no connection.

When she was 2 1/2, a tumor developed near her right knee, and her doctor had it removed. It turned out to be benign, but soon afterward she could no longer bend her right knee. Doctors discovered that bone had begun to form around the tendons and ligaments there.

Dr. Victor McKusick, of Johns Hopkins University Hospital, eventually made the diagonsis of myositis ossificans progressiva. For several years Monica was treated with an experimental drug called ethane hydroxy diphosphonate, which is supposed to interfere with the depositing of certain minerals that makes bones hard.

That drug was discontinued about three years ago. "We had no sense it was helping Monica," Zasloff said.

Monica's mother remembers clearly the day the myositis ossificans flared up. It was June 9 of last year and the family has been to King's Dominion the day before. The following day, Lydia Anderson noticed a swelling on the left side of Monica's neck.

Over the next few days, the entire left side of Monica's neck became swollen, hot, red and painful. When the swelling subsided, it left a sheet of bone deposited in its place. Soon afterward, a similar swelling developed under Monica's chin and grew to the size of a grapefruit before it dissipated and left a bone deposit.

After that, said Zasloff, "every few days, an area of Monica's body became red, swollen and painful and a week or two later a small or large piece of bone would be deposited. The result has been Monica's progressive loss of function."

In some myositis ossificians patients, Zasloff said, doctors have tried to remove surgically the additional bone deposits, but they usually grow back.

Since Monica entered the clinical center, her schoolmates have written and visited her regularly. When her class went to the Anacostia Museum, the children made a cassette tape recording of the visit and sent it to Monica. Monica sent it back with an account on the other side, detailing her life at NIH.

She has limited use of her hands but is able to participate in some arts and crafts activities. "I made a belt for my father," she told a visitor proudly one day.

Her mother has to dress, bathe and feed her, and helps Monica with some study assignments, which her school sends out regularly.

"Monica does all of the brain work herself," Lydia Anderson said. CAPTION: Picture, Lydia and John Anderson help Monica as Lydia Hatcher watches. By Craig Herndon-The Washington Post