The angiotensin-converting enzyme 2 (ACE2) protein has been proven to be a cell receptor of SARS-CoV-1 and SARS-CoV-2 for mediating entry into host cells.[50] ACE2 is an X-linked gene with a sex-specific expression profile.[51] Males also have a higher rate of smoking than females, and smoking is related to higher expression of ACE2; therefore, it might also be another factor causing more severe COVID-19 cases in males than in females.[52,53] However, whether ACE2 is linked to the clinical manifestations in patients with SARS-CoV-2 infection requires further investigation. This evidence concerns the gene ACE2 and COVID-19.