Interestingly, in a study on MERS-CoV infection of polarized intestinal cell monolayers, it was found that Caco-2 cells grown on Transwell inserts were more susceptible to viral infection upon inoculation of the virus at the apical than they were at the basolateral side, while inoculation of the virus into the stomach of a transgenic mouse expressing human DPP4 led to intestinal infection spreading to the respiratory tract and inducing animal death [43]. This evidence concerns the gene DPP4 and digestive system infectious disorder.