English: Genomic organization and expression, and key domains of four nidoviruses. The coding regions are partitioned into ORF1a (yellow), ORF1b (violet) and the 3′ORFs (blue), which also differ in expression mechanism as indicated on top. Black squares, ribosomal frameshifting sites. Within ORFs (white rectangles), colored patterns highlight domains identified in: all nidoviruses [TM2, TM3, 3CLpro, RdRp, and Zn-cluster binding domain fused with HEL1 (ZmHEL1) [114] - light and dark blue], large nidoviruses (ExoN, OMT - orange), certain clades (NMT, NendoU - red; ronivirus-specific domain (RsD) - light green; arterivirus-specific domain (AsD) - dark green). Genomic organizations are shown for Beluga whale coronavirus SW1 (corona), gill-associated virus (roni), Nam Dinh virus (mesoni), and porcine respiratory and reproductive syndrome virus North American type (arteri).