A Gene Codex is a Xel'Naga device use to store the genetic information of countless species.
This crystal-like storage unit holds key information of the primal DNA of all species created by the Xel'Naga including their own. With it, they kept tabs on the evolution of the most promising of their creations. With a fraction of the information in a Gene Codex combined with Planet Forge technology, identical worlds could be made at the subatomic level and possibly beyond.
After the prophecies of the return of the Fallen One, all Gene Codices where either strategically scattered across time and space to avoid being altered by their enemy and found at the right time by their creations.