Further Information: | COVID-19 is an infectious disease caused by SARS-CoV-2, a new member of the same coronavirus family that caused SARS and MERS. Entry of SARS-CoV-2 into human host cells occurs through binding of surface unit S1 of its spike protein to the cell receptor angiotensin-converting enzyme 2 (ACE2). ACE2 is ubiquitous and widely expressed in the lung, heart, kidney, vessels, brain, gut, and testis at different levels. ACE2 is also mostly bound to cell membranes and normally less present in circulation [1]. It was found that increased ACE2 expression in nasal epithelium and lung is associated with higher rates of SARS-CoV-2 infection and severity of COVID-19, respectively [2,3]. A new observation indicates that increased ACE2 concentration in circulation may be associated with increased incidence and fatality rate of COVID-19 [4]. In addition, SARS-CoV-2 invasion degrades ACE2 on cell membranes and may increase soluble ACE2 concentration in circulation [1]. The importance of measuring ACE2 level in various samples is emphasized by the complicated SARS-CoV-2-ACE2 interaction under normal (healthy) and abnormal (pre-existing diseases and viral infection) conditions. |