 Double s 301 made their unofficial debut under DSP Media in 2008 with the EP, "UR Man," as "SS501 Special Project Group" in the absence of SS501 members, Kim Hyun-joong and Park Jung-min.In 2015, the three members signed under CI Entertainment, who rebranded the group as Double S 301 before releasing their official debut album, Eternal 5, the following year.Member Hyungjun enlisted for mandatory military service in 2017, temporarily putting the group on hiatus.