SARS-CoV nsp13 has been shown to interact specifically with the cellular RNA helicase DDX5, which is involved in coronavirus RNA synthesis. In addition, the cellular helicase DDX1 is recruited to RTCs, and the effects of DDX1 expression knockdown indicate that it might be an essential cofactor for coronavirus RNA replication and transcription. Interestingly, the helicase activity of nsp13 is enhanced 2-fold by nsp12 through direct protein-protein interaction, suggesting that interaction of these proteins in a functional RTC improves the efficiency of viral RNA synthesis.