The maintenance  of such a large genome of CoVs may be related to the special features of the CoV RTC, which contains several RNA processing enzymes such as the 3′-5′ exoribonuclease of nsp14. The 3′-5′ exoribonuclease is unique to CoVs among all RNA viruses, probably providing a proofreading function of the RTC. Sequence analysis shows that the 2019-nCoV possesses a typical genome structure of CoV and belongs to the cluster of betacoronaviruses that includes Bat-SARS-like (SL)-ZC45, Bat-SLZXC21, SARS-CoV and MERS-CoV. Based on the phylogenetic tree of CoVs, 2019-nCoV is more closely related to Bat-SL-CoV ZC45 and Bat-SL-CoV ZXC21 and more distantly related to SARS-CoV