TMPRSS2 and viral infectious disease: For instance, the recent finding that TMPRSS2-negative cells may use cathepsin B/L7 for alternative viral entry complicates the identification of SARS-CoV-2 target cells.23 By tracing the viral infection process at consecutive infection stages in primates via single-cell transcriptomic analysis, we stand to better understand the host–virus relationship, which might help find potential druggable targets.