Stopping a movie clip from playing

How can I stop a movie clip from playing on a rollover event? I chose the mc and added the following action script but it didnt work
on (rollOver) {stop();}

I am trying to create a news bar which runs over the screen from left to right, so I used a mask and a tweened clickable text (set of buttons embeded within a movie clip) and I want the mc to stop when the user rolls over it.
Thank you