B1A4's Sandeul is the voice behind "Grown-Up Diary" for the 'Start-Up' OST.

The music video follows the past and struggles of Seo Dal Mi (played by Suzy) and Nam Do San (Nam Joo Hyuk). "Grown-Up Diary" is a calm, acoustic track about growing into an adult but still having the heart of a child.

Listen to Sandeul's "Grown-Up Diary" above! Have you been watching 'Start-Up'?