DNA microarrays (or DNA chips) have been the most commonly used technique during the last two decades to globally monitor the cellular abundances of transcript species. A DNA microarray is a collection of microscopic DNA spots attached to a solid surface. Each DNA spot contains many thousands of copies of a specific DNA sequence, known as probes. These usually correspond to a short section of a gene generally at the 3′ end. Each microarray contains one or a few probe sets for each interrogated gene.