Because a person's DNA is the same in every cell, biological evidence from a crime scene can be compared to known samples from those involved in or suspected of a crime. Once a suspect is identified, a blood or a cheek swab sample is collected and sent to the laboratory with the crime scene evidence for DNA analysis. The laboratory analyst compares DNA profiles from the evidence and the suspect to determine if there is a match. Blood and semen are the traditional sources of DNA evidence. Police officers, however, are discovering that other evidence can be analyzed for DNA, including saliva left on a rape victim, chewed tobacco or gum left behind at a murder scene, mucous on clothing or tissues, and skin cells left on the end of a weapon used to deliver forceful blows. In all cases, DNA evidence must be properly identified, preserved, and collected at a crime scene to maintain its integrity and to maximize its value as an investigative tool. The use of DNA database systems is discussed, and the Combined DNA Index System is described as a system to help crime laboratories quickly compare convicted offender and crime scene DNA profiles. Implications of the DNA profiling of convicted offenders and including such data in computerized systems are examined.