DNA suitable for the most common variety of forensic testing is found within cells in the human body. The human body is rich with cellular material as there are trillions of cells that make up the human anatomy. Most of our body parts contain cells with nuclei. It is within the nucleus of the cell that the DNA resides. This so-called nuclear DNA is the focus of most forensic typing.
There are areas in and on the body that do not contain nuclear DNA. Within blood, it is the white blood cells that contain DNA, as red blood cells are actually anucleated. The cellular material in hair shafts does not contain an appreciable amount of nuclear DNA. If hair evidence is to be used for DNA typing, there must be root material attached to provide the nuclear DNA. Finally, some skin cells are anucleated or may be dried out to the point that the nuclear DNA is no longer accessible.
Probably the most common sources for forensic DNA evidence testing are: blood, semen, saliva, and hair roots. However, bones, teeth, urine, and even fecal material may provide nuclear DNA typing results. A non-sperm cell contains approximately 6 picograms of nuclear DNA (a picogram is a trillionth of a gram). 500 picograms will pretty routinely provide a large amount of DNA information. This equates to only about 83 cells. The original technology used for forensic DNA typing, RFLP, required roughly 50 nanograms of DNA (over 8,000 cells).
With the current sensitivity of PCR-based tests using highly refined kit chemistry, forensic investigators can be creative with evidence types. Examples of interesting samples that can be subjected to DNA analysis include: cigarette butts, chewing gum, drinking glasses, pens, doorknobs, steering wheels, food with bitemarks, bitemarks on skin, ropes and cords, toothbrushes, gloves, firearms, sweat from clothing, jewelry, eyeglasses, and swabbings from computer keyboards.