BILLINGS, Mont. - Bob Hawke is a bear of a man who was an athlete in high school and, until a few years ago, a relatively healthy adult.

But during all the years that Hawke, 60, didn't give much thought to his health, a stutter in his genetic code was causing a dangerous buildup of protein on his brain.

As he edged past his 50th birthday, that extra protein began manifesting itself as a batch of seemingly unrelated symptoms. Hawke's hands began to shake, his coordination diminished and his muscles and memory weakened.

Hawke and his wife, Sandra, spent six years bouncing from doctor to doctor - and state to state - in search of a diagnosis.

Last year, a geneticist at the Mayo Clinic in Rochester, Minn., recognized Hawke's symptoms as signs of Fragile X tremor ataxia syndrome, a rare genetic disorder that was first described only five years ago.

A blood test confirmed there was a hiccup in the genetic code on the X chromosome Hawke inherited at conception from his mother.

The Hawkes were relieved to get a diagnosis, but their relief was tinged with despair: There is no way to treat Fragile X tremor ataxia syndrome, and there is no roadmap to show how quickly it will deteriorate Hawke's physical and mental abilities.

"This thing is so new they don't have enough people who have been through it to say in five years this is where you'll be or in 10 years this is where you'll be," he said. "Sandy and I are thinking we better get going with any vacations we have planned."

Fragile X tremor ataxia syndrome is a variation of another, more serious genetic disorder called simply Fragile X syndrome. The mutation is the most common genetic cause of developmental disabilities, according to Dr. John England, a neurosurgeon at Billings Clinic.

About 25 percent of all developmental disabilities are clearly genetic in root, England said.

Fragile X syndrome is inherited and occurs twice as often among boys as girls. About 1 in 4,000 boys is born with the disorder, England said.

Fragile X and its sister syndrome, Fragile X tremor ataxia, occur when a sequence in the genetic code of a certain gene on the X chromosome is repeated too often.

"It's a copying error," said Annie Adams, a genetic counselor at Shodair Children's Hospital in Helena.

Everyone has some repeats in the code, but too many can damage the chromosome and make it look broken when it is viewed through a microscope. That's how the disorder got the name Fragile X.

The syndrome affects girls less often than it does boys because girls have two X chromosomes. If one is damaged, the other can take over, Adams said.

Doctors first described Fragile X syndrome in the 1940s, but it wasn't until the early 1990s that researchers identified the faulty gene on the X chromosome that causes the disorder. Boys born with more than 200 of the genetic coding sequence repetitions will have Fragile X syndrome.

People with fewer than 50 repetitions are considered to be normal, but more than 50 repeats can lead to Fragile X tremor ataxia syndrome. The syndrome appears almost solely in men.

"We used to think if you had between 50 and 199 (repeats), it never really manifested as anything," England said. "What we know now - and we've only known it 10 years or so - is men who have the pre-mutation are normal until age 50."

It is thought that as many as 1 in 813 men fall into that "pre-mutation" category and could develop symptoms in their older years, England said.

"We wouldn't even have thought of this 10 years ago," he said.

The problems usually don't appear until later in life because it takes decades for the negative effects - that buildup of protein on the brain - to accumulate.

Hawke thinks he suffered his first symptoms of Fragile X tremor ataxia syndrome as long as 25 years ago, but the symptoms didn't begin to interfere with his daily living until more recently.

Hawke tires easily, so he breaks up his workday at the Depot Antiques Mall, a business he and his wife have owned for 10 years, by napping in a recliner in the back room.

His hands shake so badly that he can't write anymore, so he uses a stamp to sign his name. When he walks, his movements are uncoordinated. He can't stroll down the street with Sandra by his side because he would bump her right off the sidewalk.

But perhaps most frustrating of all, Hawke struggles to remember simple things such as the names of his employees or regular customers.

"This is the kind of business where people love to be recognized. They love to be remembered," he said. "Somebody spent $2,000 in my store last week, and I can't remember their name."

Because Hawke's health is faltering, he and Sandra have decided to sell the Depot Antiques Mall.

"He's reached a point where we are going to happily slow him down," Sandra said.

"There are women my age who could lose their husbands to a heart attack tomorrow and there's no warning," she said. "This isn't fatal, and I have that information. Bob and I have our future. There's a huge relief when you get a diagnosis when you've been drifting for a decade."

Although little can be done for Hawke, the genetic implications of his diagnosis are important for the rest of his family.

Men pass their only X chromosome on to their daughters. (Their sons get the Y chromosome.)

If the X is damaged, such as in the case of Fragile X tremor ataxia syndrome, those daughters become carriers of the genetic mutation.

Chances are the daughters won't have symptoms because their second X chromosome, inherited from their mothers, will take over. Without a genetic test, they won't have any idea that they carry the damaged chromosome.

But there is a 50 percent chance they will pass that damaged X on to their own children, and if their children are boys, they could be born with Fragile X syndrome. If they are girls, those girls could one day pass on the damaged chromosome to their children.

A simple blood test can determine if a woman carries a damaged X chromosome, and the women in Hawke's family, including his daughter, can choose whether to be tested before having children.

Hawke thinks his mother passed her damaged X chromosome on to some of her other sons as well. His late older brother was clearly developmentally disabled from birth and another brother shows signs of autism, which can also be a manifestation of the damaged chromosome.

Hawke hopes all of the women in his family consider the genetic test.

"It's just worth knowing," he said.