In Greek mythology, a chimera is a fire-breathing monster made of multiple creatures: a lion’s head, a goat’s body, and a serpent’s tail. In real life, chimeras are rare and not quite as dramatic. But they do exist.

Chimerism is a term in genetics that describes a single organism with two distinct genotypes. Some human twins can be born chimeras, and other people can become genetic chimeras as a result of medical procedures.

Chris Long, an American who had a bone marrow transplant, discovered this firsthand when his genetic tests came back showing not only his own DNA, but also that of his German donor—in his blood, cheeks, and even semen. He learned this because he happens to work in the Washoe County sheriff’s office in Nevada.

Long’s colleagues in the forensics unit were curious about just what the procedure would do to his DNA. He submitted to their requests to be a human guinea pig. In the four years since the lifesaving transplant, he’s been undergoing continual genetic testing at the office.

The forensic experts expected some alterations to Long’s DNA as a result of the transplant. The goal had been to replace his blood with his donor’s, so that would naturally affect the genetics of his blood. But they never imagined the extent of the change he actually experienced.

By now, the DNA in Long’s semen is no longer his own but his donor’s. Only the DNA in the hair on his head and chest have remained entirely unchanged. “I thought that it was pretty incredible that I can disappear and someone else can appear,” Long told the New York Times.

His colleague at the sheriff’s office expressed a similar sentiment. “We were kind of shocked that Chris was no longer present at all,” said Darby Stienmetz.

But Long hasn’t actually vanished. The changes aren’t visible to the eye, and genetic experts told the Times they believe it would be impossible to actually pass on a donor’s DNA to a child, despite the semen test results suggesting Long is gone.

Still, his case raises intriguing questions about the reliability of DNA evidence in criminal investigations and prosecutions. That is why Long’s colleagues took an interest and how they stumbled upon other situations where DNA evidence didn’t necessarily reveal the identity or even gender of the person who left it behind at a crime scene.

In one Korean case, an investigator encountered a victim’s body with two distinct sets of DNA, one belonging to a man and another to a woman. It turned out that the dead man had a bone marrow transplant and his daughter was the donor. Her DNA had in part replaced his.

From a biological perspective, it’s all fascinating. But from an investigative one, these oddities expose a major problem.

If a chimera like Long, with the DNA of another in his semen, were to commit a rape, for example, genetic testing would lead police astray. Yet juries put a lot of faith in DNA evidence, some say too much. Failing to account for lab mistakes and other mixups, they consider it irrefutable proof, much more reliable than witness testimony or an alibi, say.

Someone falsely accused in that situation would find it very hard to avoid conviction given that belief in the evidence’s irrefutability. And, until now, there hasn’t been widespread awareness of the fact that some medical procedures may influence an individual’s DNA. A donor would probably never realize that their contribution had led to wrongful prosecution.

But Long’s situation, which brings existing uncertainty about the reliability of DNA evidence to a whole new level, may change that. It’s a fitting contribution to criminology from an employee of a sheriff’s office.