The principle of cDNA microarray hybridization takes advantage of the property of DNA to form duplex structures between two complementary strands. In this technique (Fig. 1 ), the cDNA probes, which are arrayed onto a glass slide and represent the sequence of known genes or expressed sequence tags (ESTs), interrogate fluorescently labeled cDNA targets synthesized from extracted mRNA. In two-color microarray experiments, the differentially labeled cDNA targets (e.g., from tumor and normal tissue) hybridize to their respective cDNA probe sequences tethered to the slide. After imaging the microarray slide for signal intensities in each color channel, the relative expression ratio for each arrayed gene can be determined. In contrast to traditional gene-by-gene expression monitoring (such as Northerns), the cDNA microarray technique is limited only by the number of genes printed on the slide and, therefore; allows the analysis of gene expression on a truly genomewide scale (see Note 1 ).