ACE2 and severe acute respiratory syndrome: The scarcity of and, most importantly, the highly heterogeneous nature of normal human bronchial epithelial (NHBE) cells with regard to their ACE2 expression and, thus, permissiveness to SARS-CoV infection [39], [40] (Tseng, C.K. et al., unpublished observation) greatly limits their use in exploring the genome-wide responses to viral infections.