On February 23, 2007, I was fortunate enough to speak to a former member of SFPD’s lab, who is aware of (and who participated in) all of the preliminary DNA work on the Zodiac letters. This person was kind enough to shed some light on some critical issues surrounding the Zodiac DNA. I am only presenting a bare bones outline of one aspect of our conversation. I may decide to expand upon this section in the very near future but for now I want to present just the following information:
 

This person stated that during his or her tenure at SFPD, they tested all of the Zodiac letters in the possession of the department for the presence of two things: saliva and cells.  The way in which they tested for saliva was by looking for the digestive enzyme amylase.  They tested the flaps and representative stamps from each envelope (i.e. since most envelopes had several stamps on them, they would only test one stamp from each envelope).  This person stated that he or she did not detect much amylase at all on the Zodiac letters.  They stated that there may have been “a little bit” present.  There was, however, much less than they would have expected to detect from the presence of human saliva.  The assays on the envelopes were described as being “pretty rigorous.”  The investigation was as complete and state of the art as possible at the time it was done.

The other thing they did was to test for the presence of nucleated epithelial cells, which are abundant in saliva and which are shed from the lining of the mouth and applied to the letter when the envelope/stamp is licked.  This person said that none of the letters or stamps the lab tested had these cells in the abundance they would expect to see if the stamps/flaps had been licked. 

Saliva is teeming with nucleated epithelial cells and is also very rich in amylase.

This person’s professional opinion after years of lab experience is that due to the very low level of cells and the essential absence of amylase activity, none of the stamps or seals of the Zodiac letters came into contact with saliva.  He/she feels that it is a reasonable inference to say that the “true Zodiac letters” were sealed with, and the stamps applied to them with, something like tap water.

In contrast to the “true” Zodiac letters, this person did state that there were two letters that were the exception to the rule and which were teeming with both nucleated epithelial cells and amylase.  DNA was easily extracted from both of these letters.  They were clearly sealed with saliva and therefore differed from the “true” Zodiac letters.  One of these was the 1978 letter.