Although mu opioids share many pharmacological characteristics, they also reveal many differences. Many approaches over the years have suggested the existence of multiple mu opioid receptors. The unique selectivities of naloxonazine, for example, provided a way of distinguishing mu₁from mu₂actions. Studies of morphine-6beta-gluruconide suggested that its actions involved yet another mu opioid receptor subtype. The cloning of a mu opioid receptor, MOR-1, provided a way of exploring this possibility at the molecular level. Recent studies have now identified a number of splice variants of this gene that appear to be important in the production of mu opioid analgesia.