If it were not for the great variability among individuals," 19th century physician William Osler observed, "medicine might as well be a science and not an art." There will always be room for art in medicine, but the advent of diagnosis and treatment based on molecular knowledge of diseases is shifting the equation decidedly toward science. Almost from the moment the Human Genome Project completed its draft sequence in 2000, the intimate genetic knowledge it conferred has been accompanied by promises of a powerful, customized form of medicine. Visionaries talk of people carrying their entire genetic sequences on personalized CDs, of medicine artfully tailored to individual anatomies, and of diagnostic tests' predicting who is likely to respond to a particular medicine, who is likely to react badly, and who is unlikely to benefit at all.

Armed with details of individual variation, biologists could parse patients  into subgroups and predict which is likely to have an aggressive or indolent form of a disease and which would respond to one drug rather than another. Allen D. Roses of GlaxoSmithKline and Duke University School of Medicine has predicted that this approach, called pharmacogenetics, "will change the practice and economics of medicine," and the popular media have picked up and amplified that message. In 2001 BusinessWeek hailed personalized medicine as an idea that "has captured the imagination of biotech futurists," and Newsweek suggested that "if pharmacogenetics works, the days of one-size-fits-all therapy could go the way of bleeding by leeches."

But underneath those extravagantly rosy and somewhat wishful predictions lie important scientific, economic, and  societal questions, beginning with one of feasibility. David Altshuler, director of the Medical and Population Genetics program at MIT's Whitehead Institute and an endocrinologist at Massachusetts General Hospital, points out that personalized medicine remains "a model, a hypothesis" of the way medical care will evolve. "The genome is going to empower all sorts of things, but it's not going to happen for 20 years," he says. Along the way, personalized medicine is likely to raise a number of prickly issues: foremost among them is the paradox that the more personalized the medicine, the less interesting the business. As numerous observers have pointed out, big pharmaceutical companies have become addicted to blockbuster drugs. Targeting a smaller subgroup of a patient population by definition focuses on a smaller market.

Theoretically, one economic advantage of personalized medicine is that clinical trials might be conducted more efficiently and with a greater chance of success when researchers can so specifically select patients for testing. But how small does the pie of potential patients have to shrink before it ceases to be economically viable? Furthermore, personalized medicine is not without social implications. A technology that identifies who will benefit from a new treatment automatically identifies who won't benefit too.