The Human Genome Project, completed in 2003, identified the sequence of the approximately 25,000 genes that comprise the human genome. Knowledge of the structure of the human genome opened the doors to studying the actual function of specific genes, which is necessary to understanding health and disease. DNA microarrays are one technique that advances understanding of the genome from structure to function. For the most part, every cell in the body contains a complete set of identical DNA. What makes a given cell different from another is that only certain genes are active, or “expressed,” within the cell. The information contained within the DNA in a given cell is transcribed into messenger RNA (mRNA), which is subsequently translated into proteins that allow the cell to exist and function. Diseased cells may fail to work properly because they do not express necessary genes, express the wrong genes, or express inappropriate amounts of a needed gene. The ability to detect these abnormalities provides researchers with key information that may be used to diagnose and manage diseases.