Non-structural protein 3 (Nsp3) contains at least seven different functional modules within its 1922-amino-acid polypeptide chain. One of these is the so-called SARS(severe acute respiratory syndrome)-unique domain (SUD), a stretch of about 338 residues that is completely absent from any other coronavirus. The SUD domain may be responsible for the high pathogenicity of the SARS coronavirus, compared to other viruses of this family.

SUD(core) exhibits a two-domain architecture. The N-terminal subdomain (SUD-N) comprises Nsp3 residues 389-517, and the C-terminal subdomain of SUDcore contains residues 525-652, also named "middle SUD subdomain", or SUD-M.

This superfamily entry is represented by the middle SUD subdomain. SUD-M binds single-stranded poly(A); the contact area with this RNA on the protein surface, and the electrophoretic mobility shift assays confirm that SUD-M has higher affinity for purine bases than for pyrimidine bases PMID:19052085.

PFAM:PF11633, INTERPRO:IPR024375,DOI:10.1371/journal.ppat.1000428,DOI:10.1128/JVI.01781-08