On April 28, 2011, Starship Entertainment announced the creation of a Sistar sub-unit called Sistar19, which includes main vocalist Hyolyn and rapper Bora.

[3] Written by producer Brave Brothers, "Ma Boy" is a medium tempo neo soul song with lyrics that express the emotions of a girl in love.

[10][11] The song entered at number 2 on the Gaon Digital Chart for the month of May 2011, with 1,372,864 downloads sold and 12,625,061 streams.

[15][16] The song placed at number 14 on the Gaon Digital Chart for the year 2011, with 2,652,474 downloads sold and 21,560,202 streams accumulated.

[17][18][19] The music video for "Ma Boy" features Hyloyn and Bora dressed as "racing models", dancing next to sports cars.