There is not always a clear difference between cult and mainstream media. Works such as Star Trek, Star Wars, Harry Potter, The Lord of the Rings, The X-Files, Back to the Future, Scream, The Notebook, Avatar: The Last Airbender, Rocky Horror, Apartment Zero, The Dark Side of the Heart, XXY, Fight Club, Showgirls, That's My Boy, Killer Bean Forever, American Psycho, Barb Wire and Mean Girls attract mass audiences but also have core groups of fanatical followers. Professors Xavier Mendik and Ernest Mathijs, authors of 100 Cult Films, argue that the devoted following among these films make them cult classics. In many cases, films that have cult followings may have been financial flops during their theatrical box office run, and even received mixed or mostly negative reviews by mainstream media, but are still considered a major success by small core groups or communities of fans.