DNA, or deoxyribonucleic acid, is the road map for cell creation in the body. Containing 46 chromosomes, 23 from each parent, DNA's genetic information tells the body how to build itself, determining everything from eye colour to body weight. Except for identical twins, all DNA in the nuclei of the body's cells is unique, much like fingerprints. But unlike prints, which can be blocked by gloves or wiped away, DNA is present in blood, semen, saliva, sweat, hair, skin cells and other body substances found at crime scenes. A buccal scrape, which looks like a toothbrush capped with a small sponge instead of bristles, is used to rub the inside of the cheek for a genetic sample. The isolated DNA sample is fed into a machine that first replicates the genetic sequences using a chemical reaction and then indexes nine different length characteristics of the chromosomes. Different people have different 'sized' chromosomal units, and a Macintosh computer attached to the sequencer measures different DNA lengths by passing the liquid through minute capillaries and a laser beam. The nine characteristic lengths ensure a near-absolute match. In the case of semen, if only one lines up, then the chances are one-in-10 the DNA came from the person who secreted the semen. But with nine categories, that one-in-10 chance rises by a factor of eight to become one in a billion.