DNA is transferred to an item of evidence through the physical transfer of biological material such as blood or skin cells. Typically, biological material that is properly collected, packaged, and stored can successfully be tested for DNA many years after the commission of a crime. However, many factors impact the ability to obtain results including the type and amount of biological material transferred (i.e. "touch" DNA generally transfers less cells than fluids), exposure to environmental or other factors that can degrade DNA, and time between when the DNA was deposited and when it was tested.

For more information on factors affecting DNA analysis, see Module 3, Practical Issues Specific to DNA Evidence.

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