"Forensic DNA testing can be done very quickly—in a few hours. Typically the protocol is PCR"—that’s polymerase chain reaction, a technique for making multiple copies of a piece of DNA—"from tiny amounts of sample, followed by gel separation of DNA size variants"
-George Church, Professor of Genetics at Harvard Medical School and Director of the Center for Computational Genetics.
These “size variants,” which are DNA chunks of different lengths, together create a unique “fingerprint” that can be used to identify an individual.
(Via Kate Becker of NOVA PBS)