DNA microarray analysis is a technique that scientists use to determine whether genes are switched on or off. If a gene is switched on, it is known as gene expression. Scientists use DNA microarrays to measure the expression levels of thousands of genes at the same time. The result is known as an expression profile. This technique is used in many areas of biological and medical research. It can give valuable information about, for example, what genetic changes are responsible for tumour growth in specific individuals, or whether the expression profile of an individual makes them suitable for a specific treatment.