Double S 301 (also known as SS301) is a sub-unit of the popular South Korean boy band SS501, composed of members Heo Young-saeng, Kim Kyu-jong, and Kim Hyung-jun. Before the group adopted their official name, they made an unofficial debut in 2008 with an EP titled UR Man, released under the name SS501 Special Project Group. Their track "U R Man" earned a lot of positive attention at the time and was declared Song of the Month at the 32nd Cyworld Digital Music Awards in March 2009. The group made its official debut in 2016 after rebranding as Double S 301 under CI Entertainment. This 2016 return was accompanied by the mini-album ETERNAL 5. This comeback was an enormous success as were the accompanying live shows, with tickets selling out within minutes. Double S 301 expanded their presence to Japan with the album Eternal S (2016) and a concert tour in Tokyo and Osaka. 2016 also saw the release of the special album Estreno, which peaked at number 3 on the Korean charts. In 2017, member Hyung-jun enlisted for mandatory military service, temporarily halting group activities. Despite this, the group released Unison Volume 1 (2017) to commemorate SS501's 12th anniversary. Hyung-jun was discharged in December 2018, but the group has not been active since his enlistment.