PROS1 and COVID-19: SARS-CoV-2, the etiological agent of COVID-19 pandemic, contains a unique insertion of amino acids (AAs), exclusively proline-arginine-arginine-alanine-arginine (PRRAR685 ↓ ) at the S1/S2 boundary of its S-protein, which is clearly absent in SARS-CoV and other related hCoVs72.