"One of the most awesome uses of the microarray is to characterize different kinds of cancer cells," Herskowitz declares. "Suppose you are treating a group of 10 women who all have breast cancer and their cancer cells look similar. If you could use a DNA chip with 5,000 known human genes and ask which genes are up and which are down in various patients' cells, you might find out that these 10 women actually have several different kinds of cancer.

"And this is incredibly important," Herskowitz continues, "because it allows you to give them very precise therapy. Assume their cancer cells fall into three different patterns of gene expression: A, B, and C. If you give all these women a particular regimen of drugs, you might find that women with type A breast cancer are very effectively cured by it, while for the others it's useless. So if you treated those women as one group, you'd say, 'Well, we had a 3 out of 10 success rate,' which people would say, 'Well, that's pretty good.'

"But it was actually better than that—3 out of 3 for the right disease, and zero out of 10 for the others. So this kind of information is going to revolutionize drug discovery," says Herskowitz, "because now you can do clinical trials in which you treat people who have exactly the same disease." At present, most of the scientists in Brown's lab do experiments both in yeast and in human cells. "It's more fun that way," says Brown.

Brown's microarrays are not the only kind available to researchers. There are several different types to choose from, including the popular DNA chips made by Affymetrix, a biotech company in Santa Clara, California. These chips have much in common with silicon chips, and the technique with which they are produced—photolithography—is normally used in the semiconductor industry. Each chip is a half-inch square of glass on which thousands of short filaments of DNA have been imprinted. The DNA filaments are synthesized from lab chemicals and represent known sequences of DNA. When a liquid that contains chopped-up genes (or mRNA) from a particular cell is poured over the chip, only those bits of genetic material that perfectly complement a synthetic filament on the chip will stick to the chip and glow. A scanner then reads out their pattern.

— Maya Pines

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