Mixtures of DNA from more than one contributor are commonly encountered. Mixtures of more than one contributor could be due to:

• Actual contribution by multiple donors during the crime.
• Presence of exogenous DNA on the substrate prior to the evidence sample being deposited.
• Contamination during crime scene processing and sample handling (collection, packaging or testing).

Any biological material (blood, semen, saliva, sweat, hair, or other cells left behind after contact) can all be mixed and found in combination with any other.

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