SBS's forensic medical drama "Sign" has enjoyed solid ratings and viewership. The drama also features a fantastic soundtrack and they've recruited f(x)'s very own Krystal to sing the title track, "? ??? / Because of Me" for the show's 5th OST release. Earlier today, Sign's 5th OST was released and you can hear Krystal shine on this track. Check out the track below and remember to support the artist by purchasing the music. Tip: Clover