ACE2 and obesity due to melanocortin 4 receptor deficiency: ACE2 is known to be expressed in proximal and distal airway epithelial cells and studies have shown that ACE2 expression is associated with aging, interferon (IFN) stimulation, electronic cigarette aerosol usage, and is further increased in patients suffering from a chronic obstructive pulmonary disease (COPD) and obesity [30,31,32,33].