It should be made clear that most DNA evidence is quite reliable, provided basic standard procedures are employed. I.e. as long as you don't switch samples or

It should be made clear that most DNA evidence is quite reliable, provided basic standard procedures are employed. I.e. as long as you don't switch samples or otherwise mess up, the results are very robust. The particular issue here is the mixed samples. The basic approach of DNA fingerprinting is to look at a suite of variable markers which, taken together, form a unique set of alleles that can positively identify an individual. It's easy to see how a mixed sample can pose a problem: if you have two contributors and 20 loci analyzed, that's over a million permutations to consider, assuming all loci are different for those samples. Making an affirmative match in this situation is much more difficult! Not only must your statistics be much stronger (to offset the permutation complexity), but you must make far more assumptions about the sample than before. Namely, you must estimate the number of contributors to - and the relative proportions of - a sample via various heuristics to arrive at a statistical metric of match reliability. It's kinda amazing that it can be done at all, but for smaller numbers of contributors, it's not that bad. The classic example is a sample that contains DNA from both the perpetrator and the victim; such 2-contributor situations seem to be well understood at this point. The main issue for reliability is when there are a large number of contributors, and particularly when very sensitive assays are used. Increasing sensitivity means you have to rely more upon amplification, and the exponential nature of PCR makes it very easy for minor contributions to be out-competed. Basically, not all DNA evidence is equal, and it's important to distinguish between more and less reliable methods. Unfortunately the vagaries of legal precedent are often have an outsize influence on what is accepted in court. The apparent reluctance by the NIST to report their findings is really what is most concerning here, not the tests per se.