In our study, we found the expression levels of ACE2 and TMPRSS2 in the oral epithelial cells of the relatively elderly people were higher than those of the relatively younger people, and the expression of TMPRSS2 in the oral epithelial cells of males was higher than that of females with similar age, which may explain the higher positive rate of SARS-CoV-2 nucleic acids in male and the elderly COVID-19 patients in the saliva test report we published earlier [12]. Here, ACE2 is linked to COVID-19.