KRT18 and infection: While we could detect virus in the brains of some K18-hACE2 mice on day 3 post-infection with high levels in the only remaining mouse on day 7, none of the founder lines had detectable virus in their brains on day 3 or 7, more accurately mirroring the human tissue tropism of SARS-CoV-2 (Supplemental Fig. 1a).