Trying to change width of mc with actionscript

Hello, noob here, not so experienced with AS, this is what I’m trying to do:

I have a rectangular movieclip, it expands and contracts (just the width) and fades from one color to another when a user clicks on any of the menu choices.

In order to avoid having to map out the animation for all of this in frames, I’d just like to do it once in code and change the parameters. So, I’ve already figured out how to fade the color of the mc, and now I’m trying to figure out how to change the width - I tried this:

onEnterFrame = function(){
mc._width += 4;

but the width expands on both sides, I just want one side of the mc to move, and the other side to stay at its current x, y. Also, can’t figure out how to make it stop expanding, I know it should be something like:

desiredWidth = 400;
if (mc.width != desiredWidth) {
mc._width += 4;
} else {

but I don’t know the correct syntax for this…

any ideas? thanks!!!