TMPRSS2 and infection: Additionally, transmembrane protease serine 2 (TMPRSS-2), endosomal cysteine proteases cathepsin L (CTSL) and B (CTSB), as well as paired basic amino acid cleaving enzyme (PACE aka FURIN) can act as putative priming receptors and play an essential role in catalyzing the S-protein for an effective attachment to ACE-2 at the early stages of cellular infection [29, 34, 35].