NCT U is a South Korean boy group formed by S.M. Entertainment in 2016, the first sub-unit from the main group NCT. The group's lineup is rotational, with the members changing each promotional cycle. The members are all part of the pre-debut group SMROOKIES created by S.M. Entertainment. Taeyong, Ten, Jaehyun, and Mark were introduced in 2013, while Taeil and Doyoung were introduced in 2015. The first six members to debut with NCT U were Taeil, Taeyong, Doyoung, Ten, Jaehyun and Mark.