If DNA evidence fails to prove that an accused person raped an alleged rape victim, does that prove that the accused person could not have raped the alleged victim? Nope. Lack of DNA evidence is not automatically exculpatory. Really, the vast majority of crimes do not have DNA evidence. Is it possible for a rape to happen and for no useful DNA evidence to be left behind? Most assuredly. I can think of a number of scenarios in which that might be the case; the obvious one is when a rapist uses a condom. Is it possible for no conclusive DNA evidence to be left behind even if the rapist(s) doesn’t use a condom? This is a little trickier; rape with an object, or by an assailant who shaves his body hair, or in a “drug-assisted” situation (i.e., GHB, Rohypnol) are just a couple of scenarios in which this might be the case. Other circumstances in which rape may have occurred without DNA being present in the victim include the assailant having had a vasectomy, failed to ejaculate, or is azoospermic (a shmancy word that means his semen does not contain spermatazoa). If DNA evidence fails to prove a connection between a group of suspects and an alleged rape victim, does that prove that no rape took place? Not at all. It may, however, mean that the proverbial net needs to be thrown wider in order to obtain a different pool of suspects. You’ve said that “Mitochondrial DNA (from hair samples) and epithelial DNA (from shed skin cells on clothing, for example) takes significantly longer to process. We have come along way with DNA analysis; a sperm sample used to take 3 months to analyze, and we can now do it in 48 hours. However, mitochondrial and epithelial samples still take as much as 3 weeks just to replicate an adequate sample to test.” Can you explain the significance of this? Is it possible for the mitochondrial and epithelial DNA to have different results than the initial DNA analysis? Nuclear DNA is what most people think of when it comes to evidence — that’s what comes from blood, semen, pulled hair (with root bulb, which is where the nuclear DNA is) and buccal (cheek) cells. Mitochondrial DNA can come from bones, teeth or cut hair … and it has to be replicated by PCR (polymerase chain reaction) most of the time in order to have an adequate sample to look at. The difference comes in with mDNA in that it is matriarchal in nature; that is, your mDNA is virtually identical to your mother’s, grandmother’s, etc. Without a matriarchal sample for comparison, mDNA is not particularly useful. Furthermore, if two potential assailants are siblings, their mDNA will be nearly identical.