A DNA microarray technology utilizes the collection of microscopic DNA spots attached to a solid surface. DNA microarrays are used to estimate the expression levels of large numbers of genes at the same time or to genotype different regions of a genome. It is a tool used to determine whether the DNA from a particular individual contains a mutation in genes like BRCA1 and BRCA2. The chip consists of a small glass plate encased in plastic. Some companies manufacture microarrays using methods similar to those used to make computer microchips. The DNA copy that is made, called complementary DNA (c DNA), is then applied to the microarray. The c DNA binds to complementary base pairs in each of the spots on the array, a process known as hybridization. A black spot indicates that none of the patient's c DNA has bonded to the DNA in the gene located in that spot. Microarray technology is a powerful technique used to compare differences in gene expression between two mRNA samples. Comparing RNA prepared from diseased cells and normal cells can lead to the identification of sets of genes that play key roles in diseases.