Angiotensin-converting enzyme 2 (ACE2) is a member of the angiotensin-converting enzyme (ACE) family of dipeptidyl carboxypeptidases. ACE2 is expressed in a variety of human tissues and has a high affinity for angiotensin I (Ang I) and angiotensin II (Ang II) receptors. ACE2 catalyzes the cleavage of Ang I to angiotensin 1-9 (Ang 1-9) and Ang II to angiotensin 1-7 (Ang 1-7), which have vasodilatory and hypotensive effects ^[1]. ACE2 plays a role in regulating blood pressure, fluid balance, inflammation, cell proliferation, hypertrophy, and fibrosis, as well as in the regulation of cardiovascular and renal function and fertility ^[2]. ACE2 is also the common functional receptor for the spike protein of human coronaviruses HCoV-NL63, SARS-CoV, and SARS-CoV-2 ^[3]. However, due to species differences, the SARS-CoV-2 virus cannot bind to the ACE2 receptor of wild-type rodents ^[4-5]. Replacement of mouse Ace2 with human ACE2 by gene editing techniques resulted in humanized ACE2 mice (hACE2) that stably express human ACE2 receptors for COVID-19 studies.

In this strain, the mouse endogenous Ace2 gene sequence was replaced with the coding sequence (CDS) of the human ACE2 gene, while retaining the sequence encoding the mouse ACE2 protein signal peptide. This resulted in the expression of the hACE2 protein under the control of the endogenous mouse Ace2 regulatory elements. In addition, an enhanced green fluorescent protein (EGFP) expression cassette was inserted into the model to allow tracking of hACE2 expression. The integration site of the hACE2 is located on the mouse chromosome X. Homozygous female and hemizygous male hACE2-EGFP mice are viable and fertile.