The Xenocytes are alien "leeches" artificially manufactured by the Highbreed and are mass produced and attached to a host, which create what they are commonly known as the DNAliens.
As they attach to their host, they take over the personalities of the people they possess and control them (hacking their DNA). Within hours, the Xenocyte will completely cover the host's bodies transforming him into a DNAlien. Xenocytes eggs are created from an egg-laying machine. The eggs hatch underwater in large artificial green rivers. After hatching, the Xenocytes are stored in boxes filled with DNAlien slime, probably to keep them fresh, considering that in the end of Max Out they were seen living in open air and many were shoved into wooden crates with seemingly no DNAlien slime it suggests that Xenocytes are amphibious lifeforms. The Xenocyte eggs were going to ship out all throughout the country, but they failed.
The idea of Xenocytes may have been originated from the facehugger aliens in the "Alien" movie series.
It has been confirmed that Xenocyte DNA was in the Codon Stream, it was sampled in Max Out, but the Codon Stream noticed that they are non-sapient, so it deleted the DNA.
Xenocytes are a reference to Starro's parasitic clones in DC Comics.
Xenocytes are also similar to the Illithid monster from Dungeons & Dragons and the Flood from the Halo Games. All three are tentacle-faced creatures that take a humanoid form by altering an existing being.
Xenocytes have the same concept as Yeerks from the Animorphs books. Both are parasites that infest the mind of a sentient being and control their body against their will.
The Brain Parasites from lego Hero Factory are similar to the Xenocytes except that they can fuse to Silicon based life forms.