Menu prototype

I’m trying to make menu prototype and I have some difficulties with reflecting current active menu item.

When I click on one menu item its background is changing x position, but how do I place it back when another menu item is clicked?

In other words how to show current selected menu item.

I’m using this kind of code:

MovieClip.prototype.menuItem = function()
   this.onRollOver = function() {
      this._alpha = 100;
   this.onRollOut = function() {
      this._alpha = 70;
   this.onRelease = function() {
      this.bg_mc._x = 10;

Thank you in advance!