TMPRSS2 and infection: Pseudotyped SARS-CoV-2 viruses expressing S proteins have been developed using various viral vectors, including HIV-1, murine leukemia virus, and vesicular stomatitis virus (VSV), and infection conditions have been optimized using cell lines that overexpress hACE2 or TMPRSS2, a serine protease that primes SARS-CoV-2 spike proteins for viral entry [1,9,10,11,12,13,14].