In this respect, it is also noteworthy that ExoN mutations in TGEV, HCoV-229E and MHV have been reported to influence viral replication and transcription (Becares et al., 2016; Eckerle et al., 2007; Minskaia et al., 2006) and that the phenotype of ExoN knockout mutants can be strongly influenced by the innate immune response of the host cell (Becares et al., 2016) .