ACE2 and chronic obstructive pulmonary disease: Moreover, it has been shown that the exposure to cigarette smoke increases the activity of both ACE and ACE2 activities in the lung in mice (Hung et al., 2016), and that the expression of ACE2 is higher in the small airways of human patients with COPD and in current smokers when compared with healthy subjects and former smokers (Alqahtani et al., 2020; Leung et al., 2020).