This group consists of viruses which have (+) sense single stranded RNA genomes. It contains the virus which has the largest known non segmented RNA genome; this is the mouse hepatitis virus (MHV) which has a genome of 31.5kb. These genomes share the structure of eukaryotic mRNA and so the viruses can use some host cell proteins during replication and gene expression which occurs in the cytoplasm of the host cell. Unlike many viruses they do not have any polymerase in the virus particle as the genome can be read directly as mRNA when it first enters the host cell.
This group of viruses express the structural proteins separately from the non-structural ones. The structural proteins are encoded at the 3’ region of the genome and are expressed from a set of subgenomic mRNAs. These viruses encode one main proteinase and between one and three accessory proteinases which are mainly involved in expressing the replicase gene. These proteinases are also responsible for activating/inactivating specific proteins at the correct time in the virus life cycle and so ensure replication occurs at the right time. There are still a large number of proteins which have been identified on the genomes of nidovirales but whose function has not yet been determined.