The nsp7:nsp8 heterodimer binds to the RNA-directed RNA polymerase (nsp12) of the human SARS coronavirus on the polymerase thumb domain facing the NTP entry channel. Binding in this position sandwiches the RNA polymerase index finger loop between nsp7:nsp8 and the polymerase thumbdomain. The nsp7:nsp8 heterodimer may facilitate the interaction of the viral RNA polymerase with additional components of the RNA synthesis machinery. The second subunit of nsp8 interacts with the viral RNA polymerase interface domain proximal to the finger domain and the RNA template-binding channel, and is not bound to nsp7 (Kirchdoerfer and Ward 2019). This is consistent with the stoichiometry of the complex between feline coronavirus proteins nsp7, nsp8 and nsp12 (Xiao et al. 2012).