TMPRSS4 and HIV infectious disease: To explore the potential impact of HIV infection on gut epithelial cells susceptible to SARS-CoV-2 infection, we investigated the gene expression profile of ACE2, TMPRSS2, and TMPRSS4 using single-cell RNA-Seq (scRNA-Seq) data sets from human SARS-CoV-2–uninfected tonsil, liver, lymph node, duodenum, and blood, as well as a published human lung scRNA-Seq data set (20).