The AC18F antibody reacts with angiotensin-converting enzyme 2 (ACE2), a type I integral membrane protein, which includes a short C-terminal cytoplasmic domain made of 22 amino acids, a transmembrane domain, and a large N-terminal ectodomain made of 740 amino acids. The N-terminal domain of ACE2 contains a catalytic zinc-binding motif, which shares a 42% similarity to the catalytic binding domain of ACE. Despite the similarity in catalytic domains, ACE2 functions as a carboxypeptidase, while ACE functions as either a peptidyl dipeptidase or an endopeptidase. In humans, ACE2 is a key regulator of the renin-angiotensin system, where it catalyzes the cleavage of the vasoconstrictor peptide angiotensin II to angiotensin 1-7. Expression of ACE2 can be found on endothelial cells throughout the body, such as in the kidney, heart, and lungs. In the airways leading to the lungs, ACE2 provides the main entry point for SARS-CoV-2 (2019-nCoV). Once the SARS-CoV-2 Spike Protein binds to ACE2, viral fusion with the host cell membrane can occur, resulting in viral RNA genome delivery.