DNA which is also known as the genetic fingerprint is present in many biological substances such as blood, semen, skin cells, bone, and saliva. Because people leave DNA on many items such as cigarette butts, facial tissues, and eyeglasses investigators gather the DNA and use it for evidence in criminal investigations.
             DNA has been able to identify perpetrators years after they have committed the crime. One such example of this was when detectives in Baltimore County, Maryland, reopened a case and pursued leads in a sexual-assault homicide case that had remained unsolved for 19 years. In an effort to identify DNA evidence from the victim, officers requested the microscopic slides, made during the autopsy, from the chief medical examiner's office. When the DNA came to the laboratory the staff extracted sufficient DNA from the slides to produce a satisfactory DNA profile to assist in solving the case.
             Although DNA evidence obtained many years ago can contain forensically valuable DNA material there are some enviromental factors that exisit at a crime scene that can affect DNA and render it useless. Examples of these enviromental factors include heat, sunlight, moisture, bacteria, and mold. Also if DNA is not properly obtained and secured the DNA evedience has a good chace that it would degrade. Also when identifying, obtaining, and handling DNA evidence the officers must make sure that they do nothing to contaminate the material. Contamination can occur when the evidence comes in contact with anothers bodly fluids by actions such as sneezing, coughing, or touching. So detectives who obtain DNA must be very careful otherwise if they mistreat it DNA could very easly be rendered useless.
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