The coding sequence of the first translated exon of the mouse gene (exon 2) was replaced by an expression cassette containing a loxP site, a modified human ACE2 cDNA, loxP site, woodchuck hepatitis virus posttranscriptional regulatory element (WPRE), SV40 polyA signal, and FRT-flanked PGK-neomycin resistance gene via CRISPR/Cas9 technology. The human ACE2 cDNA was modified to use the mouse Ace2 signal peptide and codon-optimized for a mouseThe first loxP site interrupts the 5â untranslated sequence of the mouse Ace2.