Chapter 1 Applications of pathology /Fig 1.06.jpg

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Gene expression microarrays were developed in the mid-1990s and have become a powerful tool to study global gene expression. Real time polymerase chain reaction (RT-PCR) is used to generate cDNA from mRNA extracted from test and control samples. The test and reference cDNAs are labelled with different fluorochromes, in this case represented by the red and green circles. These samples are then competitively hybridized to an array platform that comprises representations of known genes or expressed sequence tags (ESTs) which have been spotted onto a solid support, usually glass or nylon. The presence of specific cDNA sequences in each sample can then be determined by scanning the array at the excitation wavelength for each fluorochrome, with the ratio of the two signals providing an indication of the relative abundance of the mRNA species in the two original samples. Although spotted microarrays are still in use today, the market is now dominated by one-colour platforms such as the Affymetrix GeneChip in which a single sample is hybridized to each array. Gene expression microarrays have been used in numerous applications including identifying novel pathways of genes associated with certain cancers, classifying tumours and predicting patient outcome.