Sil was a alien-human hybrid created by the Search for Extraterrestrial Intelligence program.
Known as test subject S1L ("Sil"), she was formed from the fertilization of a human ovum with alien DNA. Sil was raised and monitored in a controlled lab environment until the scientists studying her attempted to kill her. Escaping her confines, she broke out into Los Angeles where she evolved and fell to her alien instinct to find a suitable male and mate; becoming an intelligent but hyper-sexual predator.
She is the main character and antagonist of the film Species.