Upon infection, SARS-CoV-2 enters its target cells via: (a) binding of its spike protein (S) to angiotensin converting enzyme 2 (ACE2); (b) activation through proteolysis of the viral S protein catalyzed by the cellular transmembrane protease serine 2 (TMPRSS2); and (c) fusion of SARS-CoV-2 virus with the host cell membrane. The gene discussed is ACE2; the disease is infection.