Mouse Ease on axis

I looked at the ‘mouse follow with easing’ tutorial. http://www.kirupa.com/developer/mx/followease.htm

Can someone help me with modifying the code. I’m not sure how to restrict it to one axis and set boundaries. I only want it to move along the x-axis.

 
onClipEvent (load) {
 _x = 0;
 _y = 0;
 speed = 5;
}