The DNA chip is a device created by analogy with electronic chips, and is intended for the simultaneous detection of multiple specific DNA sequences. DNA chips are used in biomedical research for studying gene expression and mutation analysis. The supports themselves are made from glass, silicon or plastic chips. DNA is immobilised on a chip by microprinting and chemical cross-linking at fixed locations (spots) , and each spot contains an equal amount of synthetic DNA fragments with unique sequences. In other array-based hybridisation technologies complementary DNA fragments are attached to the surface of microscopic beads. Modern DNA chips can simultaneously measure the expression level of tens of thousands of genes in humans and detect about one million mutations. The use of microarray for gene expression profiling is based on the following. Active gene expression in a tissue leads to accumulation of its matrix RNA (mRNA). First mRNA is extracted from a tissue sample, and then so-called complementary DNA (cDNA) is synthesised from the mRNA using the enzyme reverse transcriptase; cDNA is much more stable and easy to manipulate than mRNA.

The resulting cDNA is labeled with fluorescent or radioisotopic labels. The content of individual cDNAs in the sample must be proportional to the copy number of the initial mRNAmatrices and, consequently, to the expression level of corresponding genes. cDNA is applied onto the microarray chip with DNA spots. Each spot contains a specific DNA sequence corresponding to the coding region of a gene. The cDNAs bind to the chip if they find their complementary sequences in DNA spots. The higher the amount of a particular cDNA in the solution, the more it binds to the corresponding spot. Then, a special scanning device determines the content of cDNA in each spot of the microarray chip, and the software correlates it with the gene corresponding to the spot. Such DNA microarray results in a matrix of dots whose intensity is directly proportional to the activity of corresponding genes.