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Alleged Golden State Killer Busted by DNA. But Are Tests '100%' Accurate?

An alleged serial killer has been charged by police in California for crimes reported decades ago. Here's what the science can and can't tell us.

Related: The Real CSI

DNA testing and a genealogy database reportedly provided police with a breakthrough in the infamous Golden State Killer case. Joseph James DeAngelo, 72, was arrested Wednesday by the Sacramento Sheriff on charges of murder. Police link the alleged Golden State Killer to 12 homicides, 45 rapes, and 120 home invasions in the 1970s and ’80s in California.

Investigators sequenced DNA from crime scenes that had been stored for decades and plugged the genetic profile of the suspected assailant into an online genealogy database. Officers found distant relatives of DeAngelo’s and traced their DNA to his front door, The New York Times reported.

Although police say they have a range of evidence in the case, they are touting the strength of what they claim is a "100% DNA match." In response to social buzz around the case, we wanted to explore the concept of a 100% DNA match. What certainty can DNA testing bring to a case?

Short answer: Not really 100%. The longer answer: While DNA is extremely good at identifying individuals, mistakes have happened. Accurate identification is dependent on a number of factors, including the quality of the DNA sample, the number of genetic markers analyzed, whether the sample was prepared properly, and the ability of those doing the analysis to interpret the results. (Learn more about forensic science.)

A DNA match identifies the potential source of the DNA, but it says “nothing about the activity involved in the DNA coming to be where it was found,” Sue Pope, a DNA expert, told The Guardian.

DNA (deoxyribonucleic acid) is found in many of the cells in our bodies, from heart to skin, blood to bone. It contains genetic information that helps determine physical characteristics such as eye color. DNA is made up of billions of letters of the genetic code and, amazingly, 99.9 percent of that code is exactly the same in every human. The 0.1 percent of the code that’s different in each of us is what make us genetically unique (with the exception of identical twins, who typically start out life with the same DNA but experience small changes through random mutations as they age).

DNA testing uses a dozen or more genetic markers in the code to identify the important similarities or differences between people. The more markers used, the greater the accuracy, but also the cost of testing. The probability of the DNA profiles of two unrelated individuals matching is on average less than 1 in 1 billion.

A sample can be from any part of the body, since the DNA is the same. In the lab, the DNA is separated and millions of copies are made using a method called polymerase chain reaction or PCR. These DNA copies are cut into chunks at specific locations to make them easier to analyze and thus create a DNA fingerprint. The result is then compared to the DNA fingerprint from another sample to see if they match.

Even advanced DNA testing, which allows the recovery of minute traces of DNA, cannot prove how the DNA got to a crime scene. We are constantly shedding skin cells or leaving traces on door handles, on money, or even by shaking a hand. DNA can hang around in a room for months or blow out the window and be taken miles away.

“You could detect a single cell on a knife blade, but that doesn’t mean anything—it might have arrived there long before the crime or been transferred there by chance,” John Butler, special assistant to the director for forensic science at the National Institute of Standards and Technology told Frontline.

In the case of the alleged Golden State Killer, as in all other cases, it is up to the prosecution to present a coherent argument that convinces a judge and jury of a defendant's guilt. Juries are asked to weigh the sum of evidence in each case, and to interpret the facts as best they can within the confines of the law. It's not an easy job. DNA evidence can provide valuable information—though there are some limitations.

Note: This story was updated on April 27 at 4:30 pm ET with information about the genealogy connection.