This repeat is found as four tandem repeats in a family of bacterial membrane proteins. Each repeat contains two transmembrane regions and a conserved tryptophan.

1: New Knowledge from Old: In silico discovery of novel protein domains in Streptomyces coelicolor. Yeats C, Bentley S, Bateman A; BMC Microbiol 2003;3:3-3. PMID:12625841