Sil is created by merging human DNA with that of alien DNA. She grows to be what appears to be perfectly normal teenage girl with nothing visually out of the normal, except the rate of the growth. Once she reaches sexual maturity, however, her alien needs take over her desires to reproduce. She escapes the laboratory that's been her home since birth, and now she looks for a perfect mate from the human male populace, who are somewhat less robust than Sil as her skin becomes hard and her features alien and her sexual lust puts her in a frenzy that few can survive.
Sil had a genetic duplicate stored in the cold storage. After the incident with Sil, the project was reopened and the duplicate was defrozen to better understand the species. Laura Baker looked after the codename Eve (Extraterrestrial. Vulnerability. Experiment.). Eve, too, escaped the lab and eventually found another hybrid to mate with. Eve did turn against the male hybrid and was killed, but a daughter, Sara, was born after her death.