If convicted, the individual's sample goes into a national database and stays there.

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Faced with the problem that DNA gathered at the scene did not match any sample in the national DNA database, they tried what is known as "ancestral DNA profiling".

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What's more, Nikon's Scene Recognition system draws upon the 420-pixel RGB color 3D Matrix Meter for outstanding exposures under a variety of lighting conditions by integrating a database of tens of thousands of sample images.

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Criminals are often not on the database if they were convicted before 1994, when sample-taking became routine.

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If a suspect was not tried, or was acquitted, the sample had to be destroyed and the profile removed from the database.

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In a sample of 3.6 million anonymised records taken from the database at the university, they found almost 33, 000 with "borderline" results which would normally require a re-test, and 3, 700 whose results suggested more strongly they had undiagnosed diabetes.

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Matthews was arrested after the sample was matched with a family member whose DNA had been collected for the national database.

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