Until the mid-1990s, studies of gene expression were limited to measuring transcription from one or a few genes. But then a tool arrived that changed all this, allowing the study of hundreds or thousands of transcripts at a time. This technology — the expression microarray — has revolutionized many areas of biology, from basic research to the understanding and treatment of human disease.

The study that brought microarrays to the attention of most researchers was published in 1995 by Patrick Brown and colleagues at Stanford University in California. They used an automated method to print 45 Arabidopsis thaliana cDNAs onto a glass slide. This 'array' was probed with a mixture of fluorescently labelled cDNAs that were derived from the reverse transcription of mRNAs extracted from a tissue sample. The amount of hybridization — and, therefore, the level of expression — was determined by measuring the intensity of fluorescence for each gene.

Although this was considered important in terms of the numbers of genes that could be studied, the fundamental implications for interpreting gene-expression patterns were not immediately appreciated. These became clear from later studies, such as a paper published in 1998 that used microarrays to monitor the expression of 800 genes throughout the yeast cell cycle. Subtle changes in overall gene-expression patterns over time were revealed that could not have been detected by other methods, even if dozens of genes had been studied.

By allowing different cell types to be accurately distinguished on the basis of expression patterns, microarrays have also had a huge impact on medical research. In 2000, an impressive early example of this was reported, in which microarrays were used to study the expression of more than 8,000 genes in 65 human breast tumours. The authors generated 'molecular portraits' of gene expression that allowed them to distinguish between different classes of breast tumour and to identify two new categories that had been overlooked by traditional classification tools. They were also able to identify expression patterns that predicted the response to chemotherapy. In this way, microarrays have paved the way for new methods that allow the accurate classification and diagnosis of disease, and also indicate the most effective treatment strategies.

Since 1995, the microarray has swiftly changed from being regarded as a new, cutting-edge technology to being almost ubiquitous in biological research. The study of expression patterns across an entire genome has moved from an unattainable dream to an exciting reality that has fundamentally altered biology and medicine.