Given the stable viable phenotype of the ExoN domain knockout mutants in MHV and SARS-CoV, it is remarkable that similar mutants could not be recovered for two distantly related CoVs: human CoV-229E (HCoV-229E) and transmissible gastroenteritis virus (TGEV) (Becares et al., 2016; Minskaia et al., 2006) .