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.

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