In contrast to replication, coronavirus transcription includes a discontinuous step during the production of sgmRNA. This process, unique among known RNA viruses, is a hallmark of the order Nidovirales and ultimately generates a nested set of sgmRNAs that are 5′ and 3′ coterminal with the virus genome. These sgmRNAs all include at their 5′ end a common leader sequence, whose length ranges from 65 to 98 nt in different coronaviruses. This common leader sequence is present only once at the very 5′ end of the genome, which implies that sgmRNAs are synthesized by the fusion of noncontiguous sequences,  the leader and the 5′ end of each mRNA coding sequence, called the body (B).