ACE2 and COVID-19: A significantly higher affinity was observed between modified S protein of SARS-CoV-2 and ACE2, almost 10 to 20 folds compared with S protein of the previous SARS-CoV (Di Filippo et al., 2020; Wrapp et al., 2020), which might explain the high susceptibility of human-to-human transmission in the spread of COVID-19.