For Carole Kushnir, a test that she took out of simple curiosity about her genetic make-up revealed a double risk of cancer. And for Karen Durrett, it led to her meeting a father she had never known - as will as bringing to light a potentially fatal condition. The two women are among 180,000 people who have paid to have their DNA analysed by 23andMe, the world's biggest private genomics company, to assess more than 200 genetic traits and health risks.
That number of customers is expected to soar this year after the California-based company, which takes its name from the 23 pairs of chromosomes in a normal human cell, reduced the cost of a testing kit to $99 (pounds 61) last month. The company, which was co-founded by Anne Wojcicki, the wife of the Google entrepreneur Sergey Brin, is planning to reach a million clients by the end of this year, including in Britain.
However, the boom in business and interest in genomes and genetics has led to controversy about the commercial use of one the most crucial medical breakthroughs of recent years. For proponents, such as Ms Wojcicki, whose husband has a genetic mutation that significantly increases the risk of Parkinson's disease, this is the dawn of a new era.
For the price of a night out, individuals can learn key elements of their genetic composition and take treatment, or protect their children from hereditary health risks. Critics, including doctors, bioethicists and geneticists, are concerned that people will be overloaded with information that is difficult for them to interpret, and that the test will raise unnecessary health fears or false reassurances, lead to unneeded procedures, or cause stress about conditions that cannot currently be cured.
"There are major concerns about the private genetic testing market," Helen Wallace, of GeneWatch UK, a genetic watchdog group, said. "It is unregulated; much of the information people get is misleading or unreliable and genetic tests are a poor predictor of the big killer diseases. And there is also the danger of people receiving scary surprises - without the presence of a doctor - for which they are not prepared."
Kushnir, 69, who owns a chain of hair salons with her husband in Silicon Valley, is scornful of such concerns. "It's patronising in the extreme, particularly in the era of the internet when people go to their computer to check the symptoms if they just get a sore throat, to argue that only experts can handle this sort of information," she told The Sunday Telegraph.
To take a test, a client spits into a test tube or swabs the inside of their cheek, then sends the sample for analysis. There was no family medical history to alarm Kushnir. But to her surprise, the test she took last year identified a mutation in the BRCA2 gene, which puts her at risk of breast and ovarian cancer. "I had very mixed emotions," she said. "I was obviously not happy to discover that I had the mutation, but relieved to have the information so that I could do something about it."
After follow-up tests, a cancer specialist advised her that the threat of breast cancer could be monitored by regular examinations. Because ovarian cancer is difficult to detect early, he recommended that she have her ovaries removed - the option that she took. She encouraged other family members to take tests too. Several, including one of her sons, discovered that they had a similar genetic mutation that heightened their cancer risk.
However, recent focus group findings and professional surveys illustrate the diverging opinions over how to use these new weapons against disease. Almost all the parents surveyed said they wanted to be informed of every risk of disease for their children, even if the condition was currently untreatable. Most doctors, geneticists and bioethicists believe that only information that could lead to action should be shared.
While the American Academy of Pediatrics expressed reservations about using the tests on children, except in limited circumstances, the American College of Obstetricians and Gynecologists concluded that personalised genetic profiling is "not ready for prime time". In an effort to provide guidelines for what is still a fledgling field, the American College of Medical Genetics and Genomics is drawing up a list of "a few dozen" major conditions to search for.
As the medical community tries to apply some rules, others are forging ahead.
Durrett, 53, from Roswell in Georgia, decided to take the DNA test to try to determine the cause of some minor health problems. Her experience illustrates another side effect - the exposure of secrets that can tear families apart. She not only discovered that she had breast cancer, she also learnt that the man she had called "Dad" for 50 years was not her father.
Durrett tracked down her biological father and a half-sister, who had breast cancer. When an update from 23andMe revealed that she had an elevated risk of the same disease, Durrett went to her doctor. A subsequent biopsy found that she had cancer. "If I'd waited, it would have gone into the tissue," she said.