Coronavirus nsp14 ExoN activity was proposed to be part of an RNA proofreading machinery during coronavirus replication, and accumulating data support this role. Phylogenetically, only long-size nidovirus genomes encode an ExoN activity, and acquisition of this activity seems crucial to allow nidovirus genome expansion. The discovery of insect nidoviruses with a 20-kb RNA genome, encoding ExoN function, strongly reinforced this idea.