A Predalien, one of the more obvious results of the DNA reflex.

The DNA reflex[1][2] is the term given to the process by which the species Xenomorph XX121 inherits certain characteristics from its host organism. The process accounts for the physical and perhaps even mental[3] variations apparent in some specimens of the creature.


"While the mechanism for this change has yet to be identified, it's likely that some host gene segments are capable of exerting greater influence over the Alien species than others."
Weyland Yutani briefing on the Runner's physiological differences (from Aliens versus Predator 2: Primal Hunt)

Xenomorphs born from Corcoran Highland Goats, from Alien: Bloodlines.

The DNA reflex accounts for alterations to the Xenomorph's basic form based on the genetic makeup of the host inside which it gestates. It is not entirely clear whether the process is intentional or simply an unintended side-effect of the way in which the Chestburster develops, constructed from the host's own cells.[4] However, given the way in which the changes to the adult Xenomorph are so successfully integrated into the creature, it seems highly likely that it is an intentional process. One suggested purpose for the DNA reflex is to help the adult Xenomorph adapt to the environment into which it will be born — by adopting characteristics of another organism that is hopefully native to that environment, the Xenomorph will be better suited to its surroundings. The assimilation of host DNA apparently also helps the developing creature to "hide" from its host's immune system.[5]

Briefing about the nature of Alien Embryo development. (from Aliens versus Predator: Extinction

Embryos are thought to copy around 10-15% of the host's genetic code during their early development.[1] Among the most obvious physical traits inherited as a result of the process are general stature — whether bipedal or quadrupedal — and, in the case of Predaliens, mandibles and dreadlocks. It is unknown whether the DNA reflex leads the Xenomorph to take on other, less obvious attributes, such as any inherent genetic strengths and vulnerabilities the host organism may have.[2] It has also been theorized that the adult Xenomorph may adopt some mental traits from the host, such as general intelligence and certain instincts.[3] Examples put forward to back up this suggestion include Predaliens that have been seen to ritualistically mutilate their prey in a manner similar to Yautja.[6]

As a side-effect of the DNA reflex, Xenomorph genetic material is also transferred to the host. It is this transference that allowed United Systems Military scientists to clone a Xenomorph Queen aboard the USM Auriga; as a Queen had been gestating inside of Ellen Ripley during her time on Fiorina "Fury" 161, blood samples taken from her there and later recovered by the USM contained the infant Xenomorph's DNA. However, the immense complexity of the genetic assimilation process meant the creation of a viable Ripley/Queen clone pair took years of research and numerous failed attempts.

"Pure" Aliens

"They can spawn from any host and more pure represent the multi-species interstellar parasite that is the Alien."
Aliens versus Predator: Extinction

The reflex is more obvious in some strains of Xenomorphs than others, with the Predalien being the most obviously influenced by it's host's DNA. However, in other castes, the hosts influence is subtle or seemingly not there at all. The Praetorian is said to be the most common "pure" alien strain, being largely uniform in appearance despite a variety of hosts. Indeed, the type of hosts is often irrelevant for their creation, as drones and even some runners can evolve into Praetorians and later Queens.[7]

Behind the Scenes

While the idea of the Xenomorph adopting different characteristics dependant on its host was not seen until Alien3, the concept had actually always been a part of the creature's makeup and was first suggested during the development of AlienRidley Scott has stated that, while shaping the Alien's lifecycle, the pre-production team considered how the creature would take on a different appearance were it to gestate inside a different host.[8] The director has gone on to state that the creature in the first film is merely "the man-version of Alien".[9]


  • The DNA reflex has something of a real-world counterpart known as horizontal gene transfer, which entails the transfer of genetic material between organisms through means other than traditional reproduction. Horizontal gene transfer occurs mainly between single-celled organisms, but at least one parasitic vine, Boquila trifoliolata, seems to be capable of altering the appearance of its leaves to mimic its host (perhaps with the assistance of a microorganism, although research into this is still ongoing).
  • In the novelization of Alien: Covenant, David states that the hybridized creatures spawned by the black liquid pathogen inherit traits from their host organism, similar to the DNA reflex of the Xenomorph.[10] Indeed, it is possible this is where the species acquired its ability to likewise take traits from its host.
  • In the Alien: The Cold Forge novel, it's discovered that the Facehuggers implant a mutagenic pathogen called Plagiarus praepotens - a variant of the Chemical A0-3959X.91 – 15 pathogen used by the Engineers to create Neomorphs - into their hosts.[11][12] Even prior to this, the Dark Horse Aliens comic established that Facehuggers implant a pathogen that, when absorbed by the host's tissues, creates a tumor that then gestates into a Chestburster.[4]