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Parallel to the efforts to unravel the human genome code, techniques are currently being developed to analyse the activity of all genes and proteins in a cell population or tissue. The most advanced of these functional genomic techniques is that used to study gene expression using DNA microarrays, also known as 'DNA chips'. This allows the expression of thousands of different genes to be compared in two different samples (for example, one from a sick person and one from a healthy one). Bioinformatics is essential in this technique. The expression profiles obtained in this way can be used to characterise complex biological situations (e.g., cell division and apoptosis) and diseases. There have already been reports on the opportunities in the diagnostic work-up for leukaemias and breast cancer. There are also applications on the more basic level, such as discovering precisely how the transcription apparatus works, and finding new genes and identifying their role. The use of microarrays in medicine is still in its infancy. It is anticipated that this and similar genome-wide analysis techniques will help in the elucidation of pathophysiological mechanisms, in making diagnoses and prognoses, and in monitoring treatment. The justifiable enthusiasm should, however, be accompanied by quality control, international standardisation and a critical approach towards the interpretation of results.