Forensic samples are among the most complex encountered. Blood is best known, but other biological matrices also carry genetic information. Cheek swabs (buccal swabs) collect cells from the inside of the mouth and have the advantage of being a non-invasive sample collection compared to a blood draw. Hair, depending on the presence of the root, is amenable to DNA typing. Semen, vaginal fluids, and vaginal swabs are collected in sexual assault cases. Any surface on which biological fluids (blood, oral fluid, vaginal fluid, etc.) are deposited becomes a potential DNA source.

The initial deposition (called the primary transfer) occurs from a person to a surface. It is the deposition of blood, saliva, semen, or other biological substance directly from the body onto a surface. This process could be a victim’s blood dripping onto an assailant’s clothing, saliva on a cigarette, or seminal fluid on a bedsheet.