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In the year 2000, the first complete draft of the human genome was presented by the Human Genome Project Consortium together with Celera Genetics and made the headlines worldwide. Since then, the so-called “post-genome era” has started. The expectations towards novel technology are high, but justified. The last 5 years have seen an exponential number of publications on functional genomics. The use of microarrays for gene expression profiling, genotyping, mutation detection, and gene discovery are leading to remarkable insights into the function of thousands of genes previously known only by their gene sequence. The impact of these technologies on every day clinical practice is not yet clear, although the first clinical applications are ready to be applied in specific clinical situations. This review introduces the principle of gene chip technology and gives an overview of its current and future potential.