Generation of DBT cells stably expressing mACE2: To transfect DBT-mACE2 with high efficiency, 3 ×105 DBTmACE2 cells were transfected using the program EN-158 of Amaxa 4D-Nucleofector and 1 ug of the linearized plasmid pcDNA3.1- myc-mACE2 in buffer SG (Lonza). Nucleofected cells were seeded on 24-well plates. After nucleofection, cells were incubated for 10 min at room temperature (RT), and then the cells were added to DMEM supplemented with 25 mM HEPES, 10% fetal bovine serum (FBS, Biowhittaker / Lonza) and 1% non-essential amino acids and incubated at 37 °C. 24 h post-nucleofection, G418 was added to the culture medium, to a final concentration of 800 ug/ml, to select for geneticin resistance. The selective medium was changed every 3–4 days for 2 weeks, and the cells were cloned three times by limiting dilution (1 cell/well) in the presence of G418 (800 µg/ml). Subsequently, ten clones of DBT-mACE2 were amplified in the presence of 800 µg/ml of G418.