 In forensics, they can use DNA profiling to determine whether or not the DNA of a suspect matches DNA left at a crime scene. The way that they do this is based on sections of DNA called short tandem repeats. This is an example of a region showing short tandem repeats. The repeats are short, A-G-A, tandem, which means they're back-to-back, they're attached, and they repeat. So this individual has one, two, three, four repeats at this locus. Using gel electrophoresis, which separates DNA based on size, they can determine the length of fragments of repeats of the DNA left at a crime scene, the DNA of individual suspects, and then can match it up. The FBI uses 13 different loci of repeating fragments to determine whether or not a suspect's DNA matches the crime scene. As you can see in our example here, suspect one matches at one locus, two locuses, three loci, but doesn't match at another loci. This suspect's DNA could be excluded from being responsible for crime scene DNA because it's not a match at all, the loci. Whereas suspect two matches at all for loci that we're looking at in this individual example. Like I said, the FBI uses 13, and so this person could have committed the crime while this person could not. In an interesting case in Atlanta in 2008, an individual had DNA that matched the crime scene DNA, had images of him on a security camera that matched his build and height and weight. He kept insisting that he didn't commit the crime. It turned out when they went back and looked at the fingerprints from the crime scene, his fingerprints didn't match those of the criminal. He had an identical twin, so his DNA was the exact same as his twin's DNA, and he couldn't be excluded based on DNA evidence, so they had to use fingerprint evidence to exclude him.