ACE2 and infection: Since the ACE2 gene is located on the X chromosome in humans and mice, we elected to utilize homozygous male and female hACE2KI-NSG mice (carrying either one or two knock-in alleles, respectively) as controls for in vivo infection experiments to eliminate the possibility for varying mouse ACE2 protein expression between homozygous hACE2KI-NSG male and heterozygous hACE2KI-NSG female mice.