This short domain is found in a variety of bacterial cell surface proteins. The domain is about 60 residues in length (although previously defined as 2 copies of this domain). It usually occurs in tandem pairs. It may be distantly related to the SH3 domain [1].

1: Identification, cloning, and nucleotide sequence of a silent S-layer protein gene of Lactobacillus acidophilus ATCC 4356 which has extensive similarity with the S-layer protein gene of this species. Boot HJ, Kolen CP, Pouwels PH; J Bacteriol 1995;177:7222-7230. PMID:8522531