PROS1 and infection: Notably, the conserved tyrosine residue (Y) 756 at the downstream of the short α-helix is hydrogen bonded with the leucine at position 753 (L753), the phenylalanine at position 759 (F759), and the threonine residue at position 998 (T998) in the folded S trimer (PDB code 6VSB [Fig. 1D]), which is speculated to contribute to stabilizing the prefusion conformation of S protein for infection.