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. These are used to hybridize a cDNA sample (the target) under high-stringency conditions. Probe–target hybridization is usually detected and quantified by detection of fluorophore-, silver-, or chemiluminescence-labeled targets to determine the relative abundance of transcripts in the target sample. By hybridizing nucleic acid targets including DNA and RNA extracted from a specimen to a DNA microarray, various statuses of DNA and RNA can be analyzed, including determination of nucleic acid sequences of the targets, analysis of gene mutations, measurement of expression levels and copy number of genes, and analysis of methylation status.