Follow the mouse in AS3, please help

Hi,
so im trying to make a horizontal menu bar that goes all of the way across the stage, this will be split into 5 equal rectangle boxes and i want to have a much thinner bar that slides across the top of the menu box dependant on where the mouse is [if the mouse is on the far right box i want the highlight bar to sit above the far right box]
i only want the highlight bar to track horizontally and if possible only track when the mouse if over the menu bar.

if anyone needs any images i can add them to this, just ask

any help would be great!

Thanks in advance, Mitchell