According to the literature, members of the structural domains which comprise this superfamily correspond to the structure of the C-terminal domain of the viral protein P21, a suppressor of the host's antiviral RNA silencing response. This protein is produced by the Beet yellows virus. This domain is annotated in Pfam as a siRNA binding domain (PFAM:PF11479). However, whilst Pfam and InterPro annotate this as a single chain domain, available literature PMID:16154094 suggests that this protein is comprised of a N-terminal and a C-terminal domain. As a consequence the given name indicates which of the domains we are referring to, which in the case of this superfamily corresponds to the C-terminus.

The function carried out by each termini has not been specified