FURIN and infection: Nevertheless, the results of the study here show that furin is not only able to cleave the S1/S2 site of MERS-CoV S during transient expression of S in HeLa cells, but that it also plays a critical role during the infection of Calu-3 human airway cells, as viral titers were reduced more than 100-fold in the presence of a furin inhibitor (Figure 2C and Figure 4B).