The circulating and tissue levels of ACE2 in some diseases, such as cardiovascular and chronic kidney diseases, and also smokers with chronic obstructive pulmonary disease (COPD) are increased as a compensatory response and it may be an explanation for why some persons with underling diseases are at higher risk of COVID-19-induced mortality [253, 254]. This evidence concerns the gene ACE2 and chronic obstructive pulmonary disease.