Nonstructural protein 15 (nsp15) of the SARS coronavirus (SARS-CoV-1) binds to the replication-transcription complex (RTC) through interaction with nsp8 (Imbert et al. 2008). This interaction appears to be conserved in other coronaviruses, such as mouse hepatitis virus (MHV) (Athmer et al. 2017). nsp15 is an endonuclease characteristic for the order Nidovirales that includes the family Coronaviridae. nsp15 preferentially cleaves 3' of uridines, generating 2'-3' cyclic phosphates after cleavage. nsp15 requires Mn2+ ions for catalytic activity. Functional nsp15 is needed for production of viable virions and for viral transcription (Guarino et al. 2005, Ricagno et al. 2006, Bhardwaj et al. 2006, Joseph et al. 2007, Bhardwaj et al. 2008, Bhardwaj et al. 2012). The biological role of nsp15 has not been elucidated. It may degrade host mRNAs to shut down host translation, but so far no human or viral RNA targets have been identified.