Make LookAt only rotate X Axis

I have this script:

static var health : float = 1000.0; var player : Transform; var speed = 5.0; var chaseRange = 50.0; var dieRange = 5.0; private var controller : CharacterController;

function Start() { controller = GetComponent(CharacterController); }

function OnGUI() { GUILayout.Label("\r\n\r\n\r\n\r\nHealth:" + health); }

function Update() { if (player == null) return;

 var range = Vector3.Distance(player.position, transform.position);
 if (range <= dieRange)
 {
     health = health-5;
     
    if (health == 0 || health < 0)
    {
       Destroy(player.gameObject);
    }
 }
 else if (range <= chaseRange)
 {
     transform.LookAt(player);
     var moveDirection : Vector3 = transform.TransformDirection(Vector3.forward);
     controller.Move(moveDirection * Time.deltaTime * speed);
 }

}

And I want to make the "Tiger" only rotate X axis, because when it starts to follow me it be kinda strange.