American Top 40

American Top 40 (usually abbreviated to AT40) is a syndicated radio program. AT40 was created by Casey Kasem in 1970. 
The program is currently hosted by Ryan Seacrest and presented as an adjunct to his weekday radio program," On Air with Ryan Seacrest".
The radio show was originally a production for Watermark, Inc (later a member of ABC Radio known as ABC Watermark). It is currently a production of Premiere Networks.
Some of the songs played on "AT40" had profanity or offensive content inside them. During the Shadoe Stevens era of "AT40", 2 Live Crew's "Me So Horny" aired in autumn 1989.
Hip hop music was played on the "AT40" mostly in the late 1980s and 1990s. It continues now.
In the 1970s, disco music aired on "AT40".
No song began at #1. The closest to that happening was "Erotica" by Madonna in 1992.