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In DNA fingerprinting, scientists collect samples of DNA from different sources — for example, from a hair left behind at the crime scene and from the blood of victims and suspects. They then narrow in on the stretches of repetitive DNA scattered throughout these samples. The profile of repetitive regions in a particular sample represents its DNA fingerprint, which ends up looking a bit like a barcode. Each bar in the barcode represents one particular stretch of repetitive DNA. Since these repetitive regions are common in the genome and highly variable from individual to individual, no two people (except identical twins) will have exactly the same set of repetitive regions and, hence, the same DNA fingerprint.

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