Those three million or so letters in the human genome sequence may have seemed like a lot of information to handle. But, consider the trillions of cells in the human body, each with thousands of genes working together-in combinations changing from one cell type to another and from one moment to the next.

"If the goal of the human genome project was to define genes, an important goal of the next phase is to determine how these genes, and the proteins they encode, are connected together in large regulatory and signaling circuits inside the cell," said Trey Ideker of the Whitehead Institute. "If you think of electronics, it's like we know where the transistors are, but not how they're wired up."

Scientists are sorting all this out using gene labs on chips approximately the size of a postage-stamp, called "DNA microarrays," that can provide a profile of all the genes being expressed in a given cell. The results are helping to shed light on health conditions from infectious disease to brain disorders, scientists reported during a series of seminars on "Microarrays and Functional Genomics" at the AAAS Annual Meeting.

To understand how microarray chips work, think of the DNA double helix as a zipper that can be unzipped down the middle, splitting the molecule into two single strands. A microarray chip is covered with "unzipped" DNA sequences representing thousands of genes. To see whether a sample of genetic material contains any of those genes, researchers first process the sample to "unzip" its active genes and tag them with a fluorescent marker.

When this material is washed over the microarray chip, the single strands of DNA zip themselves up with their com
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16-Feb-2003