ò-Funaltrexamine (ò-FNA) is an irreversible (covalently bonding) opioid antagonist that was used to create the first crystal structure of the ü-opioid receptor (MOR). It is selective for antagonism of the MOR over the ô-opioid receptor (DOR) and ú-opioid receptor (KOR). Chemically, it is a naltrexone derivative with a methyl-fumaramide group in the 6-position. In addition to its MOR irreversible antagonism, ò-FNA is a reversible agonist of the ú-opioid receptor (KOR) and produces KOR-mediated analgesic effects in animals. This has limited its usefulness and contributed to the development of methocinnamox as a more selective functionally irreversible antagonist of the MOR with no significant opioid agonistic actions.