Abstract Phase I/II Synbal and the La Jolla Institute for Immunology (LJI) are combining efforts to create breakthrough new tools for COVID-19 research to enhance research and drug development for COVID-19 disease. We aim to create a state-of-the-art set of humanized mouse strains susceptible to SARS-CoV-2, via the precision knock-in of a human mini gene at the mouse locus in each of three loci. The clinical features of COVID-19 are poorly mimicked by the existing single transgenic mouse models; those strains get a milder disease. In this SBIR Fast track PHASE I/II grant, the team will humanize the ACE2 viral receptor, its co-infection protein TMRPSS2, and the main antibody clearance receptor FCGRT to create double and triple humanized strains in C57BL/6J and BALB/c backgrounds. These two backgrounds are Th1 responders and Th2 responders, respectively, and will allow modeling of the extreme variability of human pathology observed in infected patients. The multiply humanized mice strains will be superior to the single- gene transgenic models available to date; and, thus, will allow for better basic research and drug development. To validate the strains, we plan to thoroughly characterize the effects of SAR-CoV- 2 infection at the clinical, virologic, histopathologic, and immunologic levels, and publish the findings. The product strains created in this project, the triple humanized C57B6L/6J- ACE2/TMPRSS2/FCGRT mice and the double humanized BALB/c-hACE2/hTMPRSS2 and C57BL/6J-ACE2/TMPRSS2 mice will be made readily available for the academic and pharmaceutical research communities through our animal vendor partner Jackson Labs or Taconic Biosciences.