On a busy Tuesday morning the Head household is alive with activity.

    Bill and Kristen Head are trying to get their children ready to leave the house so they can take their son Caleb to pre-school.

    The process is complicated by the fact that the children, Caleb, 4, Katherine, 3, and Kariana, 7 months, all have runny noses.

    Bill, a Montgomery County police officer, runs from child to child with a handful of tissues.

    Somewhere on the way from the front door to the van Caleb gets distracted by the four-wheel, battery-powered, 4x4 truck in the front lawn.

    He climbs in and tries to start it up, but the battery has worn down. The sound of the electric motor whirs slightly then peters out. He pushes

the button. Whir. Stop. He tries again. Whir. Stop.

    Caleb has forgotten about the van and his family and their morning rush.  Seconds ago he had a destination. Now all he can think about is the truck.

    Whir. Stop.

    Bill runs back and takes Caleb's hand.

    "Come on, Caleb,'' he says.

    Now Caleb is back on track. He runs toward the van, which Kristen will then drive to Wee Folk School where Caleb goes three days a week. On other days he goes to a special education preschool at New Market Elementary. In the evenings he attends speech and occupational therapy classes that teach him to converse and do basic manual tasks.

    Somewhere inside Caleb Head's DNA structure, a single strand repeats too many times.

    In most people the strand repeats 20 or 30 times. In Caleb it repeats hundreds.

    Every day, as Caleb plays, eats, sleeps, goes to pre-school and comes home, the repeated strand causes a specific gene in his DNA to turn off.

    When this happens, Caleb's cells don't produce a certain protein he needs.

    The result is Fragile X Syndrome, the leading inherited cause of mental retardation.

    In the early '90s scientists isolated the gene that, when shut off, causes Fragile X Syndrome. They named it FMR1, the Fragile X Mental Retardation gene.

    And though Caleb's parents can't see it, can't watch the gene -- essentially a protein factory -- shut down, they know it happens.

    They see the symptoms.

    The Heads waited three years to hear Caleb's first words. Now, each sentence is a triumph. Every conversation is a hard-fought battle, but a battle with victories as well as defeats.

    "He's a blessing,'' says Kristen. "Everything that other parents take for granted, for him is such a struggle.''

    Not only does the disorder cause mental retardation, but it also causes behavioral problems: anxiety, attention deficit, hyperactivity.

    In addition, it hinders the development of motor control skills.

    So when Caleb, with much effort, learned to walk and hop and jump, the Heads rejoiced. When he struggled with writing letters, they were encouraged because they knew it was harder for him than for everybody else. When he learned to speak by imitating his younger sister, the Heads felt the same pride any parent would feel.

   In public settings, Caleb can become overwhelmed. He'll cry or throw a tantrum at birthday parties while other children are having fun.

    But at Wee Folk School, despite the more than 15 other children and a flurry of activity, Caleb remains calm.

    But he cannot stay in one place for longer than a couple minutes.

    Caleb dashes from playroom to playroom. His first stop is the computer, where he clicks away on the mouse, navigating through an educational game in which he matches foods with their first letters.

    But computer time is limited so the next child can have a turn. Caleb, forced to find another activity, runs over to the clay table. He molds his

clay into a big ball, but quickly loses interest.

    He moves to the building blocks, but they can't hold his attention either.

    He visits the box of children's books three or four times, taking out a book, flipping quickly through the pages and running off again.

    "There he goes,'' says Debbie Velnoskey, Caleb's teacher.

    While the other children tend to do activities in groups of two or three, Caleb stays to himself.

    Velnoskey says he hasn't moved beyond what they call "proximity play,'' doing activities near, but not actually with the other children.

    Bill and Kristen, who live in Frederick County near New Windsor, enrolled Caleb in the preschool so he could interact with other children his age. The Frederick County school system told them he should only be in the special education preschool, but the Heads fought for Caleb and found a school that would take children with special needs.

    "Since having Caleb, we've learned how to fight for things we believe in,'' says Kristin.

    She counts her blessings every day with Caleb. After two miscarriages, she thought she'd never be able to have kids.

    "I wanted a child so bad I wouldn't have cared at all if he was purple,'' says Kristen.

    The mutated gene occurs on the X chromosome. Girls have two X chromosomes while boys have an X and a Y chromosome. In Kristen, one of her two X chromosomes has a permutation. She doesn't suffer from the effects of Fragile X, but has a 50/50 chance of passing it on to her children.

    Fragile X occurs in one out of every 2,000 boys and one in out of every 4,000 girls.

    Neither of Kristen's girls has Fragile X Syndrome. That's another blessing.

    Katherine has been a big influence on Caleb, forcing him to come out of his shell and play with her, giving him someone to imitate and learn from.

    Kristen often wonders where Caleb would be without Katherine. It's as though the family was assembled for just this purpose. Caleb needs them. But they also need Caleb.

    "He's taught us many things,'' says Kristen. "Mostly he's taught us about unconditional love.''

    Kristen thinks she's fortunate because she knows what it's like to raise a child with a disability and two without.

    Caleb's struggle has helped her better appreciate Katherine's and Kariana's growth. What is considered normal by most parents is special for

the Heads. They know that things like walking and talking don't always come easily.

    Last week Caleb fastened the snaps on his raincoat by himself for the first time. When she saw him, Kristen jumped and clapped and cheered him on -- another victory.

    For Kristen, the moment meant everything.

    Caleb just grinned.

    Reach staff writer Stephen Snyder at 410-857-7862.