Then came anger.

“Why me?” she remembers thinking, in a refrain she found hard to shake in the coming months. “I’m the good one. It’s not like I’m sick because I have emphysema from smoking or I did something dangerous.”

The gene that will kill Ms. Moser sits on the short arm of everyone’s fourth chromosome, where the letters of the genetic alphabet normally repeat C-A-G as many as 35 times in a row. In people who develop Huntington’s, however, there are more than 35 repeats.

No one quite knows why this DNA hiccup causes cell death in the brain, leading Huntington’s patients to jerk and twitch uncontrollably and rendering them progressively unable to walk, talk, think and swallow. But the greater the number of repeats, the earlier symptoms tend to appear and the faster they progress.

Ms. Moser’s “CAG number” was 45, the counselor said. She had more repeats than her grandfather, whose first symptoms — loss of short-term memory, mood swings and a constant ticking noise he made with his mouth — surfaced when he turned 50. But it was another year before Ms. Moser would realize that she could have less than 12 years until she showed symptoms.

Immediately after getting her results, Ms. Moser was too busy making plans.

“I’m going to become super-strong and super-balanced,” she vowed over lunch with Ms. Elio, her straight brown hair pulled into a determined bun. “So when I start to lose it I’ll be a little closer to normal.”

In the tumultuous months that followed, Ms. Moser often found herself unable to remember what normal had once been. She forced herself to renounce the crush she had long nursed on a certain firefighter, sure that marriage was no longer an option for her. She threw herself into fund-raising in the hopes that someone would find a cure. Sometimes, she raged.

She never, she said, regretted being tested. But at night, crying herself to sleep in the dark of her lavender bedroom, she would go over and over it. She was the same, but she was also different. And there was nothing she could do.