Graphic: Cathleen Heard

The boy had the classic symptoms of acute leukemia--low blood counts and tumor cells circulating in his bloodstream. But the diagnosis was tentative because the tumor cells looked atypical for leukemia. So doctors extracted RNA from the cells, made cDNAs from the RNA, and incubated the cDNAs with a chip bearing thousands of single-stranded gene fragments on its glass surface. The hybridization pattern suggested, surprisingly, that the boy had a muscle tumor. After confirming the diagnosis by more traditional methods, doctors prescribed the appropriate drug therapy.

This actual case1 illustrates the emerging role of DNA chips, also called microarrays, in understanding, diagnosing, and treating cancer. The value of chips arises from the nature of the disease. When defective genes cause cells to divide uncontrollably, they also disturb gene-expression patterns. These patterns may provide clues into what's going wrong in a cell; at the very least, they...

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