TMPRSS4 and COVID-19: Prompted by the ongoing COVID-19 pandemic caused by infection with the novel coronavirus SARS-CoV-2, we leveraged our unique LUAD and normal lung tissue scRNA-seq dataset to interrogate at single-cell resolution lung expression patterns of the SARS-CoV-2 receptor ACE2, as well as TMPRSS2 and TMPRSS4, two related membrane-bound serine proteases recently shown to be crucial for SARS-CoV-2 spike protein priming upon entry [7,10].