In the United States, before the enforcement of the Hays Code, some cartoon shorts contained humor that was aimed at adult audience members rather than children. Following the introduction of the Motion Picture Association of America film rating system, independent animation producers attempted to establish an alternative to mainstream animation. Initially, few animation studios in the United States attempted to produce animation for adult audiences, but later examples of animation produced for adults would gain mainstream attention and success. Some of the most prominent animations with these mature/adult themes include Aqua Teen Hunger Force, BoJack Horseman, South Park, Family Guy, Mission Hill, and Archer, along with other adult animated television series, feature films, and animation in other forms which helped the genre expand over the years, beyond animated sitcoms.