Despite the fact that a jury wouldn’t convict O.J. Simpson the first time around, we all know from watching CSI that DNA evidence doesn’t lie. But sometimes it does?

Peter Aldhous writes in New Scientist that recently in a case in Alaska, a positive DNA match was made between semen taken from a serious sex crime and a blood sample from a known criminal?but the criminal was in jail when the assault took place, so he couldn’t have done it. The Alaska forensic team also matched the semen to another known criminal, who was already their main suspect. Since the man wasn’t an identical twin and two people cannot have the same DNA, how could this be? It turns out the jailed man had once received a bone marrow transplant from the true criminal (who is a close relative, as marrow donors have to be. They share the same last name).

When forensic scientists swab the inside of your cheek, the way they do on TV, a bone marrow recipient’s sample will contain mostly his own DNA, but it be mixed with his donor’s DNA over they years, as a result of blood flow. For complex reasons having to do with the way the fetus develops, identical twins, despite sharing the same DNA, do not have identical fingerprints.

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