and normally the odds, for DNA testing, are on the order of 16 million to one. Not absolute but, reasonable. However, as with any tech, there are corner cases. The "reasonable doubt" bit is because it is absolutely impossible to prove anything, absolutely.
BTW, as in the case of those Death Row inmates, the DNA not only showed that they weren't there but that it was someone else entirely. In some cases, not even members of the same race. Of course, proving that it was someone else and identifying them are two different issues. |