DPP4 and COVID-19: Genetic evidence, including functional studies of existing CD26 human polymorphisms and humanized CD26 transgenic mouse animal models, has directly shown that CD26 is the functional host receptor for MERS-CoV, which is specifically required for host cell attachment, entry and, therefore, productive host cell infections, as well as species restrictions [10–12] Moreover, recent structural studies predict that the COVID-19 spike glycoproteins also directly interact with host cell CD26 [3].