By engineering the base-pairing between the CS-L and cCS-B in infectious genomic cDNAs  of coronaviruses and arteriviruses, it was normally demonstrated that the discontinuous step of transcription occurs during the synthesis of the negative-strand RNA, and base-pairing between the CS-L and cCS-B is required to drive the template switch of the nascent negative-strand RNA to the leader. Additionally, the stability (free energy, ΔG) of the extended duplex between the TRS-L and the complement of the TRS-B(cTRS-B), including 5′ and 3′ TRS flanking sequences , was confirmed as a critical regulatory factor for the  synthesis of sgmRNAs.