It was just a twitch - a muscle that danced wildly on the side of her left thigh while she sat in front of the television one afternoon.
But, as Toi Mason recalls, the tiny shudder struck her with a fear so intense that she prayed for it to be still.
"I was sitting on the couch when I started to feel it move," said Toi, 21, a senior at Temple University in Philadelphia. "All I could think of was, `Oh no, please, God, don't let this be it. Please don't let this happen to me now.' "
Ever since she was a teenager, Toi has been living with the terrifying possibility that she will succumb to Huntington's disease: a rare, neurodegenerative illness that wages a slow and gruesome assault on the body and mind. The disease is fatal, and there is no cure.
Huntington's has stricken members of at least eight generations of Toi's family, which is deeply rooted in Anne Arundel County. The latest victim is her father, Troy, who was diagnosed with it at age 21. Since then, he has lost his ability to control movements, speak, eat and walk.
Because she has a parent who has Huntington's, Toi faces frightening odds: There is a 50-50 chance that she carries the gene that causes the disease. If she does carry the gene, it is statistically certain that she will develop Huntington's.
Toi could take a fairly simple test to determine if she has the gene, but she has chosen not to. Though that might seem surprising, many of the hundreds of thousands of people haunted by the specter of hereditary diseases choose not to know the future.
The number of people who face this choice is growing as scientists unravel the genetic basis of more diseases. Not all tests provide certainty. For some hereditary cancers, for example, a test can give only a percentage chance of developing the disease.
Knowing what lies ahead can help people plan their health care and make major life decisions, including whether to marry and have children. But it also can be psychologically crushing.
Nancy Wexler, who heads the Hereditary Disease Foundation, has said genetic testing can deprive people of the hope that they will beat the odds.
"Denial and hope," she said in a medical paper, "are the twin mainstays of survival in impossible circumstances."
Toi agonized over whether to be tested before deciding against it. If she learned she has the gene, she said, that knowledge would put the brakes on her life.
