ACE and ACE-2, two cell surface proteases, are important regulators of the renin-angiotensin system (RAS), which plays a key role in maintaining blood pressure homeostasis and fluid salt balance in mammals. Because of its location and specificity, ACE plays additional roles in immunity, reproduction and neuropeptide regulation. ACE exists in two isoforms. Somatic ACE comprises two highly similar protease domains whereas germinal ACE comprises a single protease domain. ACE-2 consists of a single protease domain and is an essential regulator of heart and lung function. It is also a cellular receptor for the Spike protein of SARS cornonavirus that causes severe acute respiratory syndrome.