This video describes DNA test methods and DNA microarrays. "View the full Interactive Tutorial at: http://www.phgfoundation.org/tutorials/dna/6.html A DNA ""chip"" or microarray is prepared on a small solid base such as a piece of glass or nylon divided into a grid of tiny squares. To each square is attached a different and specific piece of DNA, typically a short DNA sequence that can act as a probe for a particular gene. DNA corresponding to thousands of different genes can be accommodated on a single array no bigger than a microscope slide. A single stranded DNA sample of interest is cut up and then washed over the chip. Any sequence in the sample that matches a sequence on the chip will hybridise to it and, if the sample is suitably labeled (usually with a fluorescent tag) the pattern of matches can be visualised and analysed by computer, giving a read-out of the presence or expression level of hundreds of different sequences simultaneously. DNA chips have many potential applications in biology and medicine. For example, they can be used to look for mutations in a gene, to measure how active a set of genes is in a particular cell or tissue type, to analyse how gene activity changes over time, or to compare the genotypes or gene expression profiles of different samples, for example the genotypes of people who do or do not respond to a particular drug, or the gene expression patterns of tumours compared to normal tissue."