MFSB, officially standing for "Mother Father Sister Brother", was a pool of more than 30 studio musicians based at Philadelphia's Sigma Sound Studios. They worked closely with the production team of Gamble and Huff and producer/arranger Thom Bell, and backed up Harold Melvin & the Blue Notes, the O'Jays, the Stylistics, the Spinners, Wilson Pickett, and Billy Paul. In 1972, MFSB began recording as a named act for the Philadelphia International label. "TSOP (The Sound of Philadelphia)", also known as the Soul Train theme, was their second and most successful single. Released in March 1974, it peaked at number one on the US Billboard pop and R&B charts. "TSOP" was influential in establishing the disco sound. The track sold over one million copies, and was awarded a gold disc by the RIAA in April 1974.