"We have hypertrophic cardiomyopathy," Lisa Salberg said.
It's a dangerous heart condition that can be carried by a gene - and one that's already killed four members of Lisa's family, CBS News medical correspondent Dr. Jon LaPook reports: her grandfather, an uncle, an aunt and her sister Laurie who died suddenly at the age of 36.
People with this disease might not even know they have it. An irregular heartbeat can cause sudden cardiac death. A surgically implanted defibrillator can detect and immediately correct the abnormality.
"That's the battery and the computer that reads every one of my heartbeats," Lisa said.
It's like having a safety net.
"It's like having an emergency room in your chest 24 hours a day, seven days a week," Lisa said.
Lisa always feared she had passed the gene on to her daughter.
"We were walking into day care and she just said, Mommy, my heart feels funny," Lisa said. "And my heart sunk. I said 'Oh, God.'"
Not knowing kept her up at night.
"I was afraid to go and wake her up in the morning," she said. "I was afraid what I might find."
Fortunately for the Salbergs, the gene that causes their heart condition has been isolated. It's just one of out of 20,000 to 25,000 genes in each cell of our bodies.
"A completely new era is beginning," said Dr. David Margulies. "Soon, we're going to be able to look for every known disease-causing variant in a single individual."
A machine in Margulies' lab is looking for those variants.
How many different patients are going to be analyzed?
"There are going to be 100 patients," Margulies said.
A hundred patients … who each have 20,000 different genes and we're looking for how many genes in each one?
"In that well, we might be looking for three, in another well we're looking for 10," he said. "Miniscule fractions of their genome."
It's mind-boggling … like locating a certain grain of sand on a beach.
"A severe disease could be caused by a change in just a single letter," Margulies said. "Out of the 3 billion letters in a person's genome."
Scientists found that one mutation in one of Becca Salberg's gene when she was 10 years old.
"What did they tell you?" LaPook asked.
"They just sort of told me that I have hypertrophic cardiomyopathy like my mom," Becca said. "But everything's going to be all right since we have the proper care and stuff."
The proper care for this disease includes an implanted defibrillator just like her mom's and a medicine to keep Becca's heart from racing.
"Before, when your heart would go fast, did you have second thoughts?" LaPook asked. "Were you thinking oh man, what's going on?"
"Yeah," Becca said. "Sometimes when it went really fast, I was really nervous and I just had to take a breather and just stop for a little. Doesn't happen any more now."
With the help of genetic testing, Becca feels safe doing everything she loves - just like any other kid.