A microarray containing numerous small DNA molecules, used, for example, to analyse gene transcripts or to detect mutations of specific genes. Oligonucleotide DNA microarrays consist of thousands of short synthetic single-stranded DNA molecules, each comprising 20–25 nucleotides and all with unique sequences designed to complement and bind to specific target nucleotide sequences. Spotted microarrays incorporate much larger DNA fragments obtained from cloned DNA. Such arrays give a quick and convenient method of quantifying gene expression, by determining the total output of messenger RNAs (mRNAs) (i.e. the transcriptome) of a cell or tissue. This involves adding fluorescent labels to the mRNAs, or in some cases to their single-stranded complementary DNAs (cDNAs), and then incubating the labelled nucleic acids with the DNA microarray. The nucleic acids bind to complementary oligonucleotides on the microarray, and excess nucleic acids are washed off. The microarray is illuminated so that the labels fluoresce, and a computerized scanner measures the intensity of fluorescence at each coordinate on the microarray, and thereby the amount of bound RNA or cDNA. DNA microarrays can also be designed to detect mutations in particular genes, for example the BRCA genes involved in hereditary forms of breast cancer. An individual’s DNA is denatured, and its binding to a microarray is compared with that of normal (control) DNA on the same microarray. Any disparities between the two binding patterns will pinpoint sequences from the individual with possible abnormalities, enabling closer examination. Because of faster and cheaper DNA sequencing methods, the relative amounts of mRNAs in samples are increasingly being analysed directly using RNA sequencing, as a preferred alternative to microarrays.