To Kill a Mockingbird has been a source of significant controversy since its being the subject of classroom study as early as 1963. The book's racial slurs, profanity, and frank discussion of rape have led people to challenge its appropriateness in libraries and classrooms across the United States. The American Library Association reported that To Kill a Mockingbird was number 21 of the 100 most frequently challenged books of 2000–2009.