Here's what I know about the DNA. As Georger said above, he has posted his info many times in many places, and I will do the same, because this seems like an ideal repository for this information.
1. The current DNA sample that the FBI has comes from epithelial cells found on the clasp. Larry Carr readily acknowledged to me that they could be DB Cooper's or anyone else who handled the tie. Larry confirmed it is a "partial" sample.
However, Larry did not comment on the statements made by Calame and Rhodes in their book that the tie entered the evidentiary collection in Seattle four days after the skyjacking and no one knows where it was during that time.
Further, Calame and Rhodes write that they interviewed the four FBI agents tasked with evidence retrieval in Reno, and none remember seeing, taking, or talking about the tie.
Compounding the confusion during the retrieval, Reno PD apparently was dusting for fingerprints, according to Rhodes, and a K-9 bomb sniffing team came aboard and ate the crew's dinner sitting on the seats in First Class, according to Rataczak.
2. The best source of DNA comes from human fluids, such as blood, semen, and saliva. As a result, the eight cigarette butts left by Cooper are ideal for DNA testing, even forty years later. These butts were tested circa 2002 because Pat and Ron Forman claim they heard a KIRO-TV news report stating that the FBI had Cooper's DNA from the cigarettes.
However, Larry reported on the DZ in 2008 that he went looking for the butts to test them for DNA, and they were missing. Also lost, apparently, was the paper work from 2002 detailing Cooper's profile.