Three months after his bone marrow transplant for acute myeloid leukaemia and myelodysplastic syndrome, Chris Long of Reno, Nevada, learned that the DNA in his blood had changed and been replaced by the DNA of his donor, a German man. Four years after his lifesaving procedure, it was found that not only Mr. Long’s blood, but also swabs of his lips and cheeks contained his DNA and that of his donor. Even more surprising was that all of the DNA in his semen belonged to his donor. Mr. Long had become a chimera, the technical term for the rare person with two sets of DNA. The average doctor does not need to know where a donor’s DNA will present itself within a patient. But for a forensic scientist, it’s a different story. 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. But this is not always true as seen with Mr. Long. In another case in 2004, investigators in Alaska uploaded a DNA profile extracted from semen to a criminal DNA database. It matched a potential suspect. But the man had been in prison at the time of the assault and it turned out that he had received a bone marrow transplant. The donor, his brother, was eventually convicted.