ACE2 and respiratory tract infectious disorder: Considering that L-SIGN co-expresses with ACE2 on human lung tissue cells (Amraie et al., 2021; Chan et al., 2006) and expression of ACE2 in vivo is generally low in single-cell RNA sequencing datasets (Hou et al., 2020; Zou et al., 2020), the co-receptor role of L-SIGN may have straightforward meaning in natural respiratory infection occuring upon a tiny virus content.