SETI, short for Search for ExtraTerrestrial Intelligence was a program tasked with trying to find proof of life beyond the planet Earth. The people in charge of the program sent many different messages into outer space, including information on human DNA, a map of the solar system and population of the Earth to any that might be listening beyond the stars.
Then, the impossible happened: They received a response from an unknown source not of this world. The response was two distinct messages. The initial message was how to create a energy source using methane, and the secondary message was a new sequence of DNA with instructions on how to combine it with human DNA. Since the initial message was deemed from a friendly intelligence, they went ahead with the combining of the alien DNA with human DNA.
The result was splicing the alien DNA in human ovum, resulting in seven potential hybrids. Four didn't survive, two were stored in liquid nitrogen for future use, and one was permitted to develop under the codename "S1L" for Sil, a female hybrid which achieved development in a week. In less than two months after being created, SETI became worried with what they dealing with and attempted to abort the program to perform more research.
But Sil managed to escape, and a team was made to go after her and destroy her.