Gui script attached to prefab not displaying

I have a script attached to a prefab that gets instantiated into the scene. When the prefab is moused over, it should display a gui. But it doesn’t.

The only possibility I can think of was that you can only have one OnGui function being called at one time, as I have a different script displaying several gui elements constantly. However this link negates that:

Unity Answers: Can You Make OnGUI Calls From Multiple Gameobjects

I also verified that the monobehavior checkbox is enabled in the inspector on the gameobject this script is attached to per:

MonoBehaviour.OnGUI() Script Reference

the Debug.Log(isHover); prints 1 in the console, so OnMouseOver() is working as it should.

Please help! This is my code:

function OnGui(){
	if (isHover==1){
		GUI.Box (Rect (0,0,Screen.width,Screen.height),"");	

function OnMouseOver(){
	isHover= 1;

function OnMouseExit(){
	isHover= 0;

The function is OnGUI() with upper case ‘U’ and ‘I’.