The plus strand RNA genome of the human SARS coronavirus 1 (SARS-CoV-1) is replicated by the viral replication-transcription complex (RTC) composed of nonstructural proteins nsp3-nsp16, encoded by open reading frames ORF1a and ORF1b. Two RTC proteins, nsp8 and nsp12, possess 5'-3' RNA-dependent RNA polymerase activity. nsp12 is the main RNA polymerase, while nsp8 is thought to act as an RNA primase. nsp14 acts as a 3'-5' exonuclease, increasing the fidelity of the RTC. nsp14 also has the RNA capping activity and, in concert with nsp16, it caps viral plus strand and minus strand genomic and subgenomic RNAs, which confers stability to viral RNAs by enabling them to escape interferon-mediated innate immune responses of the host. nsp13 is an RNA helicase which is thought to melt secondary structures in the genomic RNA during replication and transcription. The plus strand genomic RNA is first used to synthesize the minus strand genomic RNA complement, which is subsequently used as a template for synthesis of plus strand viral RNA genomes that are packaged into mature virions. For review, please refer to Yang and Leibowitz 2015, Snijder et al. 2016, Fung and Liu 2019.