The nsp14 protein is part of the RTC core complex, formed by nsp12 (RdRp) and the nsp7-nsp8 processivity factor, providing proofreading and capping activities. Interestingly, nsp10 is able to enhance ExoN activity up to 35-fold in vitro, binding nsp14 either alone or in the nsp7-nsp8-nsp12-nsp14 complex. The involvement of nsp10 in coronavirus RNA synthesis was first reported from the analysis of MHV mutant. More recently, it has been shown that nsp10 acts as a cofactor of both nsp14 ExoN and and nsp16 methyltransferase (MTase) activities. Moreover, as nsp14 and nsp16 bind to overlapping nsp10 sites, nsp10 might act as a molecular switch, mediating interactions between RNA and proteins from both proofreading and mRNA capping machinerries.