The Tleilaxu secret must be in their sperm. Our tests prove that their sperm does not carry forward in a straight genetic fashion. Gaps occur. Every Tleilaxu we have examined has hidden his inner self from us. They are naturally immune to an Ixian Probe! Secrecy at the deepest levels, that is their ultimate armor and their ultimate weapon.
Attributed to: Bene Gesserit
From Chapter 11 of Heretics of Dune by Herbert, Frank