Structural domains comprising this superfamily share the structure of the NN domain of the uncharacterised protein family PFAM:UPF0261 (INTERPRO:IPR008322). Available structural information about this domain comes from the characterization of the the tomato mosaic virus (ToMV) resistance gene Tm-1, which binds ToMV replication proteins to inhibit the formation of the replication complex formation. Tm-1 is comprised by an uncharacterized N-terminal region (PFAM:UPF0261) and a TIM-barrel-like C-terminal domain. According to available literature, this uncharacterized N-terminal domain is an inhibitory domain PMID:25092327.

This structurally unique inhibitory N-terminal domain is actually comprised of two discrete structural domains, named NN and NC domain respectively. It is believed that these domains are connected by a disordered inter-domain loop. Their specific function remains unknown.