"In complete darkness we are all the same,” sang Janet Jackson. But the sage of Rhythm Nation 1814 was wrong. At the genetic and cellular level, human beings are profoundly unalike. Wouldn’t it be wonderful to know how patients will react before giving them a pill?

Enter, please, the wonderful new world of personalized medicine. The idea is simple: ascertain a patient’s individual molecular makeup, then match that to a vast database of treatments to design a regime attuned to that person alone. Happily, researchers are furiously working to realize this vision. Not so happily, the private sector is also jumping in, even though huge amounts of research and testing still need to be done. And here and there, alas, a whiff of hucksterism is in the air.

The first personal-genomics start-ups—23andMe, Navigenics, Knome, deCODEme—launched in 2007. Clients pay a fee and spit into a vial to provide a few cells. The firms assay the DNA in those cells, looking for millions of tiny hereditary variations. Last year, for example, a French research team reported that breast-cancer patients with one of the two forms of the gene HER-2 responded differently to the new cancer drug trastuzumab (Herceptin).

Problem is, such successes are rare. Scientists have established huge numbers of links between particular diseases and snippets of DNA, but in the great majority of cases, this has not yet been translated into treatments that can help cure patients. These treatments will come—tomorrow, or the day after. Meanwhile, companies are opening their doors today, hoping to build their brands on the fears of the worried affluent. Good luck to them!

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