DNA, or deoxyribonucleic acid, is the fundamental building block for an individual's entire genetic makeup. It is a component of virtually every cell in the human body. Further, a person's DNA is the same in every cell. For example, the DNA in a man's blood is the same as the DNA in his skin cells, semen, and saliva.

DNA is a powerful tool because each person's DNA is different from every other individual's, except for identical twins. Because of that difference, DNA collected from a crime scene can either link a suspect to the evidence or eliminate a suspect, similar to the use of fingerprints. It also can identify a victim through DNA from relatives, even when no body can be found. And when evidence from one crime scene is compared with evidence from another, those crime scenes can be linked to the same perpetrator locally, statewide, and across the Nation.

Forensically valuable DNA can be found on evidence that is decades old. However, several factors can affect the DNA left at a crime scene, including environmental factors (e.g., heat, sunlight, moisture, bacteria, and mold). Therefore, not all DNA evidence will result in a usable DNA profile. Further, just like fingerprints, DNA testing cannot tell officers when the suspect was at the crime scene or for how long.

Where Is DNA Contained in the Human Body?

DNA is contained in blood, semen, skin cells, tissue, organs, muscle, brain cells, bone, teeth, hair, saliva, mucus, perspiration, fingernails, urine, feces, etc.

Where can DNA evidence be found at a crime scene?

DNA evidence can be collected from virtually anywhere. DNA has helped solve many cases when imaginative investigators collected evidence from nontraditional sources (see "Identifying DNA Evidence"). One murder was solved when the suspect's DNA, taken from saliva in a dental impression mold, matched the DNA swabbed from a bite mark on the victim. A masked rapist was convicted of forced oral copulation when his victim's DNA matched DNA swabbed from the suspect's penis 6 hours after the offense. Numerous cases have been solved by DNA analysis of saliva on cigarette butts, postage stamps, and the area around the mouth opening on ski masks. DNA analysis of a single hair (without the root) found deep in the victim's throat provided a critical piece of evidence used in a capital murder conviction.