All audio visual works dealing with "serious topics" such as the Cultural Revolution must be registered before distribution on the mainland. For example, "The Departed" was not given permission to screen because it suggested that the government intends to use nuclear weapons on Taiwan. Films with sexually explicit themes have also been banned, including "Farewell My Concubine", "Brokeback Mountain" and "Memoirs of a Geisha". Warner Brothers never submitted "The Dark Knight" for censors, citing "Cultural sensitivities in some elements of the film" due to the appearance by a Hong Kong singer whose sexually explicit photographs leaked onto the internet. Films by PRC nationals cannot be submitted to foreign film festivals without government approval.