Reticulans speak of a 'gene code zero' which is sacred to them, since it represents the perfect Sectoid. This code however can only be copied using rare ancient equipment, so it is a slow method to make new dudes. The Rets are desperately trying to find out how to mate with other species to breed Ret babies, but so far were only successful in breeding hybrids. They also are looking for ways to become stronger and better-suited to Earth by improving upon their sacred code zero, which probably means it's not so sacred after all. The super-quick regeneration we have witnessed on Sirens and Sectogres seems to be the most amazing effect of all this.