A gene chip is a piece of glass or plastic on which pieces of DNA, which are referred to as "probe sequences," have been affixed in a microscopic array. Machines use such chips to simultaneously screen a single biological sample for the presense of many genetic sequences.
Because "GeneChip" is a trademarked name owned by Affymetrix, scientists tend to speak and write about "gene chips" only in reference to the chips that Affymetrix makes. Brought to market in 1996, these were the first to come into use, and they are referred to generically as oligonucleotide arrays, because the probe sequences are short pieces of DNA about 10 to 50 nucleotides long. A still broader term is microarray, which refers not only to GeneChips but to chips that use much longer probe sequences, such as the entire genes that may be contained on cDNAs. Because, Affymetrix owns a patent both on the use of oligonucleotides probes as well as on the means the company uses to affix them to the chip, microarrays that are not made by Affymetrix are manufactured by a different technique that is non-proprietary.
Using microarrays, cancerous cells can be analyzed with gene chips to discover which genes are present in the cancerous cells that are not present in healthy cells. Cells can be treated with drugs, and the gene chip can analyze those genes to determine which genes are turned on or off by the drug.
Since there are hundreds of thousands of probes on a gene chip, using a gene chip can be the equivalent of thousands of conventional genetic tests. Gene chips have therefore dramatically accelerated the pace of genetic research.