PROS1 and infection: MERS-CoV enters the host through its S protein, a type I transmembrane glycoprotein with 1353 amino acids (aa) that exists on the virion surface as a trimer.21 Subsequently, it is recognized by cluster of differentiation 26 (CD26) (also known as dipeptidyl peptidase 4 (DPP4)), which facilitates the infection of the host cells.22 SARS-CoV uses angiotensin-converting enzyme 2 as a functional receptor.23 MERS-CoV and SARS-CoV differ in their cellular selection for infection, possibly owing to their selective binding with different receptors.24