Rotate a movieclip from its center

1

I want to rotate a movieclip depending upon the mouse movement. Rotation is working also but I want to rotate it from the center of the movieclip. The registration point of the movieclip is top - left

Code

var radiance:Number = 180/Math.PI;
_root.onEnterFrame = function() 
{

        //calculate rotation
        walkdirection = -(Math.atan2(_xmouse-box_mc._x, _ymouse-box_mc._y))*radiance;
        //set rotation
        box_mc._rotation = walkdirection;

}

I cannot figure out how to do this :frowning: