In using DNA as forensic evidence, what you don't find is at least as important as what you do.

Suspects and victims don't always leave behind enough DNA to yield a specific number of markers. What you find at a crime scene is what you get. In the U.S., for example, you do not have to have 13 DNA markers to be able to go to court with it, even though that is the number of markers that the FBI stores in the CODIS database.

Let us suppose, for example, that a murder occurs. From underneath the victim's fingernails, forensic technicians are able to obtain a small DNA sample from which they can extract 12 DNA markers. They send the sample to the FBI, which runs it through the CODIS database and is able to get a 12:13 match to the DNA of a convicted rapist which was taken at the time he was convicted and stored in CODIS. All the DNA markers they have match that individual's DNA; they just don't have 13 of them.

That evidence would be admissible in court, and would be regarded as very good evidence. Yeah, yeah, the defence would likely make much of the fact that they weren't able to get at least 13 markers out of the DNA sample for comparison purposes, but it's still very strong evidence.

If, however, within that DNA sample consisting of 12 markers you found say, 9 markers that matched a sample stored in CODIS and 3 that didn't, then you can't say you've got a match on the DNA. It is not uncommon for people of the same race and ethnicity to have DNA that will be match at some locations (loci) or markers. But if you get a "no match" on even one marker, then you cannot say definitively that the DNA comes from a specific person.

That was the problem with the DNA from the McCanns' hire car: They found a tiny bit of DNA from which they were able to extract a total of 37 markers that they believed was mixed and came from more than one person. From the "known sample" of Maddie's DNA extracted from her pillow in Rothley, they had 19 markers.

Of the 37 DNA markers found in the hire car, Maddie's DNA matched 15 of them. Or looking at this in a slightly different way, the FSS knew what 19 markers of her DNA looked like and found an exact match for 15 of them in the material from the hire car. To be fair to the FSS, that could never have been regarded as a definitive slam-dunk match. But neither, in my opinion, was it as totally meaningless as the final FSS report makes it appear. I think it would have had some evidentiary value if presented in court. How much value would have been argued by dueling experts and decided by the panel of judges hearing the case if anyone had been charged.

I have never believed that the FSS deliberately set out to throw the case with regard to the DNA analysis. I do believe, though, that evidence in the file indicates that there was some sort of "preliminary" from the FSS and conveyed to the Leics police in the person of Stuart Prior at least orally. This "preliminary" report , I believe, probably indicated a much greater degree of certainty that the DNA from the hire car could have been Maddie's than the final report did.

What happened to this report I cannot say and the file does not say, but if it indeed did exist, SOMEONE on this sweet earth knows about it. The Met police need to find them and find out what that original report said. If they could do that, IMO, every penny of the money allocated for the "review" would be justified.

My personal opinion is not that the FSS was leaned on by officialdom, but that after the debacle with the LCN DNA evidence in Omagh, they were afraid to go out on a limb in any regard in an even higher-profile case. But this is the problem with that attitude, IMO: Saying nothing is not always non-controversial. All science is not 100% certain and scientists have an obligation to say so, but they also have an obligation to give their best assessment of forensic evidence when asked to do so and not try to dodge controversial conclusions. The FSS will soon be gone, and in my opinion, we will be well rid of them.