If you take a look at the San Francisco Police Department DNA report, the 1974 Exorcist letter is the only letter that year to produce any viable results, described as "cells found". The S.L.A letter was totally disregarded, and the "Citizen" and "Red Phantom" communications had no entry in comments. If Alan Keel is correct in his assertion, then the Exorcist letter would enter the classification of unlikely Zodiac correspondences - ones that had "been licked by the sender".
The only remaining communications in the DNA report subsequent to June 26th 1970 which produced any notable results, classified as "cells found", were:
 The July 24th 1970 'Kathleen Johns' letter.
 The July 26th 1970 'Little List' letter, and
 The January 29th 1974 'Exorcist' letter.
And all three had one crucial element in common - The Mikado.
The 'Kathleen Johns' letter began the trilogy by stating "So now I have a little list, starting with that woeman + her baby that I gave a rather intersting ride for a coupple howers one evening a few months back that ended in my burning her car where I found them". This would be continued two days later with the paraphrasing of two of Gilbert and Sullivan's acts from The Mikado. The first section of the 'Little List' letter pulls lines from the A more humane Mikado, where the author uses the words billiard along with crooked cues and twisted shoes.
This correspondence goes on to paraphrase Gilbert and Sullivan's As some day it may happen, performed by Ko-Ko, as did the future correspondence of the Exorcist Letter in 1974, when reciting Tit-Willow from Ko-Ko's On a tree by a river, part of Act Two.
We don't know the exact concentrations of DNA retrieved from these three communications, but it is evident that these were the only three letters classified as "cells found" - and all three made reference to The Mikado. If "cells found" could be proven as "saliva" found, and the "true Zodiac verifiable letters had not been licked by the sender" according to Alan Keel, then this could indicate that all The Mikado letters were not authored by the Zodiac Killer. An extremely hard notion to accept when we look at the handwriting and design of each of these correspondences.
If the Zodiac Killer hadn't written any of The Mikado communications, then he also wasn't claiming the abduction of Kathleen Johns on March 22nd 1970. In fact, the notion of a killer driven by the theatrical librettos of Sir William Schwenck Gilbert would be quashed entirely. It is extremely difficult to sell a story of a Zodiac Killer who didn't author any of The Mikado communications, let alone all of the 1974 communications - so the curtain call will be cancelled for this performance only.