1 'LOST') is the third extended play and the first part of The Clan series by the South Korean boy group Monsta X.

In April 2016, the group announced that they would release The Clan series, revealing the first concept as Part 1: LOST.

[4][6] The music video for the title track was released on the Starship's and 1theK's official YouTube channels on the same day, directed by the award-winning film director Dee Shin.

[4][5] Writing for Dazed, Taylor Glasby described the music video for "All In" as ambitious and provocative, "but the video almost backseats the song to a soundtrack", which Glasby also describes as "far better than the MV gives it the chance to be, particularly the crooning chorus".

[12] "Ex Girl" and "All In" debuted at numbers 118 and 159 on the weekly Gaon Digital Chart, respectively,[13][14] while "All In" and "Stuck" debuted at numbers 6 and 11 on the weekly Billboard World Digital Song Sales chart, respectively.