Editor's note: This popular story from the Daily Briefing's archives was republished on July 10, 2020.

Four months after Chris Long, a cancer patient from Reno, Nevada, received a bone marrow transplant, forensic scientists found that his body contained the DNA of a man who lived thousands of miles away, raising questions about how cases like Long's could change the way investigators solve crimes, Heather Murphy reports for the New York Times.

What providers need to know about genetic testing—and 7 other innovative technologies

The 'human guinea pig' 

When Long was diagnosed with acute myeloid leukemia and myelodysplastic syndromes, his doctor found a donor with matching bone marrow to treat him. The donor was a younger man who lived thousands of miles away in Germany.

But before the transplant took place, the implications of what Long was about to do intrigued his colleagues at the Washoe County Sheriff's Office.

Namely, they were concerned about how the transplant donor's DNA could interact with Long's—and how the mixing of DNA between two people could obscure criminal investigations, Murphy reports.

When Long shared the news of his bone barrow transplant, Renee Romero, who was in charge of the crime lab at the Washoe County Sheriff's Office, told him, "We need to swab the heck out of you before you have this procedure to see how this DNA takes over your body," she recalled.  

Long agreed to Romero's request and became a "human guinea pig," Murphy writes.

Becoming a chimera

Three months after the transplant, a blood test revealed the "weak blood" in Long's body had been replaced by his donor's "healthy blood," and "with it, the DNA it contains," Murphy writes.

And four years later, DNA tests from a swab of Long's cheeks and lips revealed that Long's body contained both his and his donor's DNA. Meanwhile, another test revealed DNA in Long's semen had been entirely replaced by the donor's, Murphy reports. 

The only samples collected that did not contain any DNA from Long's donor were samples from his chest and head hair, according to Murphy.

The tests showed that after the operation, Long became a chimera, or a person with two sets of DNA, Murphy reports. Tens of thousands of people get bone marrow transplants every year and become chimeras, and it's not harmful, according to Murphy.

Andrew Rezvani, the medical director of the inpatient Blood & Marrow Transplant Unit at Stanford University Medical Center, said, "Their brain and their personality should remain the same."

The lack of harm means that most doctors don't need to know where a donor's DNA will show up in a chimera patient, Murphy reports.

However, "for a forensic scientist, it's a different story," Murphy writes. "The assumption among criminal investigators as they gather DNA evidence from a crime scene is that each victim and each perpetrator leaves behind a single identifying code—not two."

Chimerism and crime … and fertility

Brittney Chilton, a criminalist at the Sheriff's Office forensic science division, found that chimerism has misled investigators in the past.

For instance, in 2004, investigators in Alaska found a potential suspect by uploading DNA to a criminal DNA database, but then learned that the suspect was in prison when the assault occurred. The investigators later found the man had received a bone marrow transplant from his brother, who was later convicted of the crime.

Similar instances complicated Yongbin Eom's efforts to identify a victim of a separate crime, Murphy reports. In 2008, when Eom, a visiting research scholar at the University of North Texas Center for Human Identification, attempted to identify the victim of a traffic accident in South Korea, the DNA results suggested the victim was female, but the body appeared male. After finding both male and female DNA in the victim's kidney, Eom learned that the victim had previously received a bone marrow transplant from his daughter.

But cases like Long's also raise another question, Murphy reports: If a person with chimerism has a child, will the condition be passed on to the baby? Most health experts say this is impossible. "There shouldn't be any way for someone to father someone else's child," according to Rezvani, especially since a donor's cells should not be able to create new sperm cells.

As for Long's case, Mehrdad Abedi—a doctor at the University of California, Davis, who treated Long—agreed, saying the reason Long's semen was taken over by his donor's DNA is only because Long had a vasectomy. However, forensic scientists plan to continue investigating the case.

Still, Murphy writes, "Everyone who has reviewed Mr. Long's case agrees on one thing: He is a living, breathing case study of one, and it's impossible to say how many other people respond to bone marrow transplants the same way he did" (Murphy, New York Times, 12/7).