FIG. 15.2 A harmful mutation. (a) The enzyme alpha-1 antitrypsin (α1AT) normally inhibits the activity of elastase, which breaks down elastin in the lung. (b) Cigarette smoke inhibits α1AT activity and increases the risk of emphysema. (c) A mutation in α1AT called PiZ is harmful because it reduces α1AT activity and also leads to increased risk of emphysema.