A compound attribute that is the ratio of amount of angiotensin-converting enzyme 2 (human) in blood to amount of neprilysin (human) in blood. The trait is Ontology of Biological Attributes entry OBA_2055774 (ACE2/MME protein level ratio in blood). Also known as: the ratio of amount of angiotensin-converting enzyme 2 (human) in blood to amount of neprilysin (human) in blood.