PROS1 and infection: The high doses of SARS-CoV-2, which are required for the monocyte infection are in line with the alternative, i.e., ACE2- and S protein-independent entry mechanism mediated via the interaction of virus-associated cyclophilins and monocyte-expressed CD147, since even the combined amount of CypA and CypB present in SARS-CoV-2 virions is naturally lower than the amount of the S protein.