DPP4 and infection: To verify the loss of NS4band/or NS4a expression by these mutants, human A549 cells stably expressing the MERS-CoVreceptor DPP4 (A549DPP4) were infected with MERS-CoV mutants at a multiplicityof infection (MOI) of 10, and protein lysates were harvested at 24 and 48 h postinfection(hpi) to assess protein expression by Western blotting.