Structural domains comprising this superfamily share the structure of the C-terminal beta sandwich domain found in NigD-like proteins which are mostly found in Bacteroides species. These proteins range between 234 and 260 amino acids in length and possess an N-terminal lipoprotein attachment site. NigD is a protein found in the Nig operon that encodes a bacteriocin called nigrescin (NigC) and is believed to be the immunity protein for this bacteriocin operon since the start codon of nigD is located downstream and overlaps with the end of nigC. This is based on the fact that genes encoding immunity protein are usually located immediately downstream and co-transcribed with the bacteriocin gene PMID:19531060.

The function of this C-terminal beta sandwich domain is yet to be determined.