Structural domains comprising this superfamily share the structure of the family transcription anti-terminator antagonists UpxZ, which inhibit transcription of heterologous capsular polysaccharide loci in Bacteroides species by interfering with the action of the UpxY family of transcription anti-terminators. As antagonists of polysaccharide locus-specific UpxY transcription anti-terminators, the UpxZ proteins exert a hierarchical level of regulation, insuring that only one of the multiple phase-variable capsular polysaccharide loci per cell characteristic of this genus is transcribed at a time PMID:20547868.