ACE2 and infection: This homotrimer plays a critical role in infection (22), and together with its interaction with ACE2, promotes fusion of the viral and cellular membranes allowing internalization of the virus for its intracellular replication (20, 27) and causing cell viral infection mainly in the lung, where ACE2 is mainly expressed in type 2 pneumocytes (28) and causing a decrease in the number of ACE2 receptor molecules on the cell surface (15).