TMPRSS2 and infection: Canonically, TMPRSS2 is the protease responsible for infection within the lungs [29,55,56] yet SARS-CoV-2 has been found in tissues that do not express TMPRSS2 leading to the discovery of secondary proteases involved in the S1/S2 cleavage within cells as opposed to the surface fusion TMPRSS2 is involved in.