The video game "Parasite Eve" is based on the acclaimed Japanese novel "Parasite Eve" released in 1995. Plot-wise, the video game serves as a sequel to the book, referencing various events therein while also stating that Mariko Anzai, the girl whom Eve had chosen to become her host in the novel, is Aya's biological mother. The game was produced by Hironobu Sakaguchi and directed by Takashi Tokita of Square. During development of the game, Square decided to use New York City as the setting after it had been previously considered for use in "Final Fantasy VII". "Parasite Eve" was Square's first game to be rated Mature by the ESRB. In contrast to previous Square titles, the development team for "Parasite Eve" consisted of both Japanese and American staff members, with a large part of the production taking place in the United States. Different concepts for the game's opening were considered, including different designs for Aya and Melissa transforming into Eve on stage during the opera. Book author Hideaki Sena did not know the title's plot until it was completed, since the game was a collaboration between Square and his publisher.