Mike Rodelli wrote in his book, 'The Hunt for Zodiac:The Inconceivable Double Life of a Notorious Serial Killer', that Alan Keel, Criminalist at the San Francisco Police Department, San Francisco, California from 1996 to 1999 "revealed that there were two letters in possession of the department that, in contrast to the "true" Zodiac letters, had abundant saliva and DNA-containing oral epithelial cells on them, that DNA was easily extracted from these two letters, and that the DNA extracted from these two letters matched between them. These two letters were considered forgeries, since the "true" Zodiac verifiable letters had not been licked by the sender." Mike Rodelli added that "In contrast (to earlier communications), Keel analyzed two other letters, one of which is the 1978 forgery, and finds that this letter and one of the 1974 letters are loaded with saliva and cells. He then easily extracts DNA from both of these letters using the more primitive DNA technology of that time and finds that the DNA matches between those two letters, thus proving that one person sent both."
For those who resolutely argue that the Exorcist letter is a genuine Zodiac communication, may have to accept the authenticity of the 1978 letter too. According to all the above observations, these two communications are inextricably linked by one sender. In early 2018 Vallejo police Detective Terry Poyser, who has worked the Zodiac case for four years, said his agency has submitted two envelopes (in late 2017) that contained letters from the Zodiac Killer for a type of advanced DNA analysis that previously had not been available in the case. Poyser declined to identify the lab, but said it would attempt to obtain a full DNA profile from saliva on the envelope flap and stamps. He said he expected to have results back from the lab as soon as in the next few weeks, and almost certainly by summer. Sacramento Bee.
Tom Voigt recently began a forum thread on the Zodiac Killer message board entitled Vallejo Police Department DNA Update, stating "The lab work has turned into more lab work. VPD is still at it. The hunt continues..." Why are they still hunting for viable DNA when apparently a full profile has been secured previously? Even if law enforcement have little confidence in the Exorcist and 1978 letter, these would have provided the best options to run through a genealogy website 18 months ago and the results would have been in by now had anything fruitful been generated. Unless of course, you already know who forged both of these letters. I see no benefit in not seeking profiles from these two letters (generated from the 1978 letter and another, if different to suggested) to enter into a familial DNA program. If the familial search threw out a notable family tree, or the lineage of a recognized Zodiac suspect, then it would add credence to the previously doubted Zodiac communication.
For the 60% of people who voted on this site, believing the 1978 letter to be from the hand of the Zodiac Killer, the question you should be asking is, why did the the 2000 San Francisco DNA report determine the 1978 correspondence to be "not an authentic Zodiac letter" if they hadn't already obtained a full DNA profile to make such an authoritative and conclusive judgement? The only other reasonable conclusion, is that they already knew who the author was.