Hannah and Andrew are busy stacking Legos. They're very organized. She has the red and green stack, he has the yellow and blue. When they hold them up together, hers is longer, so she shortens it. Now they're just alike.

The two are, in fact, incredibly compatible.

After a while, they decide to play their favorite game. Andrew runs out of the room and Hannah sprawls on the carpet, an arm outstretched.

"Father," she gasps hoarsely. "Please help me."

Instantly, Andrew is at her side. "Don't worry, Mother," he says earnestly. "I'll save you!"

Around the house, their brothers and sister stop what they're doing to listen and smile.

The older siblings call the game "Father-Mother." Between Lego-building adventures, biking and general kids play, Andrew and Hannah play "Father-Mother" a couple of times a day.

The game has a simple premise and a million variations. Either "Father" or "Mother" is in grave danger. The other one will, somehow, ingeniously and at great personal risk manage a daring rescue. They'll hug and laugh and thank each other, then run off to play with something else.

The older kids wish it was that simple. That fathers and mothers could always rescue each other.

Rebecca halfway believes it. She's convinced that her dad will be able to raise enough money for Huntington's research that they'll find a cure. She's not sure it will be in time to help her mother. But she's pretty sure he'll be able to protect her and her little brothers and sister from the disease.

She just wishes he'd hurry.

Sources of information about Huntington's disease

There are many sources for information about Huntington's disease.

On the Web, three sites will provide phone numbers and links, as well as their own wealth of information:

• Huntington's Disease Society of America, www.hdsa.org.

• Hereditary Disease Foundation: www.hdfoundation.org, call 1-310-458-4183.

• CureHD Foundation: www.curehd.org, phone 1-801-227-0737.

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Project editors: Wendy Ogata and Ray Boren.