The function of the S-layer, a regularly arranged structure on the outside of numerous bacteria, appears to be different for bacteria living in different environments. Almost no similarity exists between the primary sequences of S-proteins, although their amino acid composition is comparable. S-protein production is directed by single or multiple promoters in fron of the S-protein gene, yielding stable mRNAs. Most bacteria secrete S-proteins via the general secretory pathway (GSP). Translocation of S-protein across the outer membrane of Gram-negative bacteria sometimes occurs by S-protein-specific branches of the GSP. O-polysaccharide side-chains of the lipopolysaccharide component of the cell wall of Gram-negative bacteria appear to function as receptors for attachment of the S-layer. Silent S-protein genes have been found in Campylobacter fetus and Lactobaccillus acidophilus. These silent genes are placed in the expression site in a fraction of the bacterial population via inversiion of a chromosomal segment.