NA is a powerful investigative tool because, with the exception of identical twins, no two people have the same DNA. In other words, the sequence or order of the DNA building blocks is different in particular regions of the cell, making each person's DNA unique. Therefore, DNA evidence collected from a crime scene can link a suspect to a crime or eliminate one from suspicion in the same way that fingerprints are used. DNA also can identify a victim through the DNA of relatives if a victim's body cannot be found. For example, if technicians have a biological sample from the victim, such as a bloodstain left at a crime scene, the DNA taken from that evidence can be compared with DNA from the victim's biological relatives to determine if the bloodstain belongs to the victim. When a DNA profile developed from evidence at one crime scene is compared with a DNA profile developed from evidence found at another crime scene, they can be linked to each other or to the same perpetrator, whether the crime was committed locally or in another state.
DNA evidence in the form of saliva, blood, skin tissue, hair, and semen are often recovered from crime scenes and can be crucial to the investigation of sexual assaults and other violent crimes. For example, during a sexual assault, biological evidence such as hair, skin tissue, semen, blood, or saliva can be left on the victim's body or at the crime scene. In addition, hair and fiber from clothing, carpet, bedding, or furniture could be transferred to the victim's body during an assault. This evidence is helpful in proving that there was physical contact between an assailant and a victim. DNA properly collected from the victim, crime scene, or suspect can be compared with known samples to place the suspect at the scene of the crime. If there is no suspect, however, a DNA profile of the crime scene can be entered into the Federal Bureau of Investigation's (FBI) Combined DNA Index System (CODIS), which allows agencies to match DNA profiles with other profiles entered into local, state, and national databases to identify a suspect or link serial crimes.
As with fingerprints, the effective use of DNA as evidence may require the collection and analysis of elimination samples to determine whether biological evidence came from a suspect or someone else. When investigating sexual assault or rape cases, it may be necessary to obtain an elimination sample, such as a blood or saliva sample, from the victim's relatives or consensual sex partner to account for all of the DNA found on the victim or at the crime scene.