Accordingly, our infection model uses ACE2-GR mice produced by deleting 71 kb of the mouse genome that encodes the mouse Ace2 protein in C57BL/6 background embryonic stem (ES) cells and subsequently inserting the 65.8-kb syntenic ACE2 segment of the human genome to generate ACE2-GR mice (Fig. 1A). Here, ACE2 is linked to infection.