Immediately downstream of the N gene stop codon, there are two overlapping, essential RNA structures consisting of a bulged stem loop (BSL) and a hairpin-type RNA pseudoknot (PK), which are structurally and functionally conserved in all betacoronaviruses. An intriguing  property of the BSL and the PK is that, because they overlap by 5 nt, they cannot simultaneously fold up completely, which has led to the speculation  that each element may adopt alternate configurations, acting as a molecular switch that operates at some stage of RNA synthesis.