This evidence indicates that for successful SARS-CoV-2 infection, cellular ACE2 and TMPRSS2 are necessary (Figure 1), and male sex hormones may contribute to the infectibility of the host (a recent clinical study reported that among 191 COVID-19 inpatients, 62% were male; among non-survivors 70% were male [21]). Here, ACE2 is linked to COVID-19.