Based on their ACE2 expression and permissiveness to productive SARS-CoV infection, 18 out of a total of 26 clones established exhibited an array of varying intensities of ACE2 expression and permissiveness to SARS-CoV, ranging from intermediate-to-low levels, whereas the remaining eight clones revealed increased ACE2 expression and susceptibility to SARS-CoV infection, when compared to their parental Calu-3 cells (data not shown). Here, ACE2 is linked to severe acute respiratory syndrome.