The uncoupling of sg mRNA transcription in vivo has also been achieved in an arterivirus, Equine arteritis virus, where mutation of two different proteins that contain zinc-binding domains, nsp1 and nsp10 (neither is the RdRp), caused specific defects in the DTS mechanism of sg mRNA transcription (van Marle et al, 1999a, b; Tijms et al, 2001) .