Horses are subject to "equine recurrent laryngeal neuropathy", a disease of the axons of the recurrent laryngeal nerves. The cause is not known, although a genetic predisposition is suspected. The length of the nerve is a factor since it is more common in larger horses, and the left side is affected almost exclusively. As the nerve cells die, there is a progressive paralysis of the larynx, causing the airway to collapse. The common presentation is a sound, ranging from a musical whistle to a harsh roar or heaving gasping noise (stertorous), accompanied by worsening performance. The condition is incurable, but surgery can keep the airway open. Experiments with nerve grafts have been tried.