<s>[INST] <<SYS>> This is a medical question of type : causes <</SYS>> What causes Hallermann-Streiff syndrome ? [/INST] What causes Hallermann-Streiff syndrome? The genetic cause of Hallerman-Streiff syndrome has not been identified. It reportedly typically occurs randomly for unknown reasons (sporadically), most likely due to a new spontaneous (de novo) mutation in the affected individual. </s>