It's a really interesting case. They did a DNA test about a decade back on the stamp from one of the Zodiac letter envelopes and it did not match Allen. Some have speculated as to why that might be the case, but who knows.
http://www.sfgate.com/crime/article/...ct-2784268.php
What's the most interesting is that the PD keeps the case "open and active" to this day.