The model for this family detects a homology domain of about 40 amino acids. Member proteins always have a least two tandem copies and as many as seven. The spacing between repeats as defined here usually is four residues exactly. This repeat is named for a tripeptide motif HAF found in most members. Some members proteins are found in species with no outer membrane (archaea and Gram-positive bacteria) while others have C-terminal autotransporter domains that suggest that the repeat region is transported across the outer membrane. This domain seems likely to be an extracellular protein repeat.