1991 Scientists at the California-based biotech company Affymax produce the first DNA chips. Affymax takes advantage of photolithographic techniques similar to those used to etch circuits onto computer chips to build their DNA probes, base by base, onto a silicon wafer or glass slide. By selectively covering and exposing the growing stacks of nucleotide bases, the scientists can control what sequences they lay down on the array. Researchers then flood this chip with fluorescently tagged DNA-isolated from a bacterium or from your blood, for example-and look to see which sequences stick. Because they know the exact sequences of the DNA probes on the grid, the researchers automatically know the sequences of the fluorescent fragments bound to them. DNA that doesn't find its match on the chip is simply washed away.

Affymetrix, a spinoff company dedicated to gene-chip technology, is established in 1993. Today the company offers a variety of gene chips, including human DNA arrays and chips for identifying HIV strains or detecting mutations in cancer genes. The chips are dense, sporting up to 400,000 different DNA probes in an area the size of a thumbnail. But their price tag-up to $2,000 per single-use chip-puts these arrays out of the reach of many basic researchers.