I have the following code attached to a GameObject with a GUITexture attached, however when I click on the button, the OnMouseUp function is not called. What's wrong? As far as I can tell, the docs say this should be possible.

function Update () { //Keep button in same relative location

    var width = Screen.width/13;
    var height = Screen.height/9;
    guiTexture.pixelInset = Rect(Screen.width-width*2, Screen.height-height*4, width, height);
}

function OnMouseUp () {//Button clicked

    print("button clicked");
    Application.LoadLevel(1);
}